I don't know how to advise you to get prescription medications online such as the ones that would benefit Norbert, and typically the sites that you can get prescription drugs from are super sketchy and actually illegal. They usually import the medications from pharmacies in Canada, Mexico, etc. where the stuff is available over-the-counter. So I don't advise that.
The best way to go about this is to either find a different vet, one that is either an experienced reptile specialist or one who is actually a Certified Reptile Vet. You may have to drive a couple of hours one-way, but it's so well worth it. I cherish my CRV of 15 years, he's amazing. So that's one way of approaching this.
The second way to go about this, and probably the first thing you should try to save you time and money, is to go online and find professional journal articles and articles from reputable reptile magazines, groups, associations, etc. discussing the different treatments for Fatty Liver Disease, especially any that actually list the dosage instructions of each. Print as many as you can find out, and bring them to your vet (I didn't realize that your vet is a General Vet, I assumed he was at least an "exotics" vet, not that an "exotics" vet is much better if any better than a General Vet). If your vet knows that you put in the time and research then he's more likely to do the same. it's actually his job to put in the time and research, not your job, that's what you're paying him for. And I would think that a General Vet who knows very well that he has no special education or experience in the treatment of reptiles/Bearded Dragons would just automatically do the extra research to educate himself on what reptile specialists and Board Certified Herpetologist Vets are doing to treat Bearded Dragons with Fatty Liver Disease.
I'm a little disappointed that your vet, who knows very well that he has done absolutely zero continuing education in Herpetology and who has never had ANY specialized education or training (such as an internship, a fellowship, or being mentored by a Certified Herpetologist) in reptile medicine at all (why would he, he chose to be a General veterinarian because he wanted to treat dogs and cats, nothing wrong with that at all until you start trying to treat reptiles with potentially life threatening diseases) told you that the only way of treating Fatty Liver Disease in a Bearded Dragon is to try to get him eating more greens and less live insects. That's not cool at all. In fact, even the very few General Veterinarians and "Exotics" Veterinarians that I know of personally who have attempted to treat a reptile who they have seen as a patient have all at the very least contacted a Certified Reptile Veterinarian or a very experienced reptile specialist veterinarian for a consultation and advisement on the diagnosis and the treatment plan, and they have all sent the reptile's medical chart to the reptile specialist that they were working with so that they could read the reptile's medical history, exam notes, test results, etc. and develop a treatment plan to advise the General or Exotics Veterinarian of. The few times I've seen this or been involved in this ALL worked this same way. So I'm very surprised that your General Veterinarian didn't do the same and contact a reptile specialist for advisement. (In addition to having 3 dragons currently, 1 a rescue with severe MBD who I adopted at 8 months old, and having my first dragon from a month old until he passed away at 12 years and 10 months old, and having a Green Iguana from 2 months old until he was almost 12 years old when I rehomed him, I have volunteered at an Exotic Rescue who specializes and only accepts reptiles, amphibians, and birds for about 7 years, and I am their Supervisor of Health and Medicine, so on the occasions where we have had a critical reptile or bird brought in to the Rescue in the evening or at a time when our regular Certified Reptile Vet or Certified Avian Vet was not available and we've had to take them to the local 24/7 Animal Hospital, which employs several General Veterinarians and a few Exotics Veterinarians who have some specialized experience, each time I've been involved in this they have done whatever they could to stabilize the reptile or bird, run any basic tests that they can like x-rays, fecal testing, blood work, swabs, etc., and given fluids, oxygen, pain meds, etc., all while trying to contact any of the reptile or avian specialists they have on-call or on a consultation basis).
Anyway, I think what you should try to do is take some professional research on the treatment of Fatty Liver Disease in to your General Vet and try to convey to him that you'd really like to be proactive about Norbert's care and try some of the FLD treatments that Certified Reptile Vets are currently using on a regular basis and that they are getting positive, long-term results from, and if he still says that he "Isn't comfortable prescribing anything for Norbert's FLD" or something similar, that's when you ask him if he could please recommend either a Certified Reptile Veterinarian or a very experienced Reptile Specialist Veterinarian that you could take Norbert to, even it they are a couple of hours away.
Unfortunately it's becoming extremely common for veterinarians (and human doctors for that matter) to not want to listen to their patients at all and totally disregard anything and everything they request and simply just say "No" and that's it. We all must be advocates for not only our own health and medical treatment, but also for the health, welfare, and medical treatment of our children and our pets (who I very much consider my own children). Norbert should have many happy, healthy years ahead of her with you and the rest of her family, and you should not let an uneducated/inexperienced Veterinarian steal those years away from you when there are so many available treatments out there that are inexpensive, safe, and effective for Fatty Liver Disease in Bearded Dragons...
If you would like, I can contact my own Certified Reptile Vet and see if he can send me some professional journal articles and other information on the current treatments they are using in Herpetology for Fatty Liver Disease in Bearded Dragons, and I could then email you the files so that you can print them out to take to your General Vet. My CRV is a great guy, I've known him for the past 15 years and he's actually called members on here for me before to give them advice, listen to their stories, and he's also found several local Reptile Specialist Vets for members here. So I know he'd be more than willing to at the very least send me some journal articles and information. He has to do Continuing Education Courses all the time to keep his Board Certification active, and I know for a fact that he has done many graduate classes at both Penn State and University of Pennsylvania School of Veterinary Medicine (that's where he went to Veterinary School originally) that dealt with current treatments for FLD, Gout, and certain common cancers in reptiles, so I'm sure he has a lot of information that he could send me.
I'll shoot him an email right now and he'll most likely get back to me with at least a yes or no pretty quickly I'm sure. I'll keep you posted.