You can get a digital kitchen scale at Walmart, they have a bunch of different ones, all that will read the appropriate ranges, and cost between $15-$25. I use one from there that cost $15 and I find it very accurate compared to my certified reptile vet's expensive, digital, medical scale.
It should not matter one bit if a fecal sample is "too dry", that has nothing to do with a float test or a gram stain and microscopy. He really does need to get the fecal testing done ASAP, as he cannot afford to lose anymore weight at all, us he's not getting adequate calcium or Vitamin B1 or D3 intake, and at his age this is going to result in bone density issues like MBD, vitamin B1 deficiency, which has the same end results as MBD, and severely stunted growth. Most parasites and/or worms eat any and all nutrition that the host animal is eating, so she could potentially be losing everything you're giving her. Any fecal sample that is solid, whether wet or dry, should be fine for all tests, and even a runny sample can be tested by gram stain and microscopy, even if a float test can't be done. Usually worms and coccidia are very visible under a microscope if gram stained.