Walmart. Under $10. If you're only getting one and you have a sub-adult to adult, I'd get a 41qt or bigger, just for a little extra room. A baby or juvenile would do well in a 12qt.
The ones from Walmart almost all of them have the respective covers within reach and they are included, just find the right size and attatch it to the tub.
You will also have to poke, drill, burn holes depending on how humid it gets in the tub. Some people say they only make 2-4 holes if they are using racks. I have holes surrounding my whole tub just for added air flow.
Okay that sounds good. I have a baby ball python and baby colombian red tail in the same tank but they are seperated down the middle. I just want more room for my BP. The UTH is on my tank and when I switch to tubs is it okay to have a heat lamp on it because the UTH is adhesive and is stuck to the tank