You can get the thermometer right at walmart or on amazon.com, and even in pet stores but it will cost more there. It doesn't have to be a specific one, just a digital probe thermometer. I like the ones that give you an in and out reading so you can get your basking temp and cool side with one thermometer.
The reason you don't house together is because they are solitary animals and don't do well with others. As babies they can be okay, but theres still a risk. They see toes as food which is how missing limps happen. They are territorial so they will fight when together. They can be fine for years and one day just snap. When you see them laying on top of each other, its not them being cute, its being dominant. Once determined which one is the dominant one, he will get the best lighting (the laying on top of one another), all the food etc and eventually the other one will just wilt away. Starfire is not going to be lonely, if anything she's happier because she has her territory back to herself. Two males together will fight to death, a male and female doesn't work because the male will constantly try to breed the female, and males are mature sooner than females so they will breed too young and hurt the female, the constant trying to breed will stress her out causing other health issues. Sometimes, but very very rarely, two females will be okay together, but can still snap at any time. It happens all the time and the only real chance of two females tolerating each other is if they are clutch mates and been together since birth. If you try to put a new female in with star fire, she will probably fight for her territory. It only takes a split second for disaster to happen. It just not worth the risk.
Most people will tell you...if you can't afford two enclosures, don't get two beardies. (i know you never said you couldn't afford it or anything so I'm not saying that to you lol)
heres an article on why you don't house together.....
viewtopic.php?f=6&t=154908
Im really sorry about Godzilla, but now you can focus on Starfire and give her everything she deserves!