most people mount them inside the cage, on top. yes they work with a thermostat. I've always herd you can mount them on the side of a cage also, but never seen anyone do it. I can snap some pics tomorrow if you want, but nothing hard to picture, just a panel mounted on top.
reason you don't see them for sale that often is because once you get them, there nothing better to switch to and i've never herd of someone needing to replace one. all my panels are from pro-products, give em a call and they will tell you exactly what panel you need for your cage. expect around 100+ bucks shipped tho.