Where can I buy a UVB light for my yellow bellied slider?

Any good pet store should carry them. They provide vitamin D, needed to metabolize the calcium from the blocks. BTW, a lump of plaster will work just as well as those blocks, at less cost. You can also give the turtle vitamin D in its diet, as by feeding liver.

i would go to a pet store asap and get one. if u put it off too long he'll die. also spend time reading a ton of animal care websites and other info sites to make sure u know all u can about him to take proper care of him. basic care u need to know first: tank size- about 20 gallons. u can find one on the internet for a reasonable price. water depth- about 16–18 in water temp.- 72–80°F food: some types of water plants, vegetables, insects, and fresh fish he will also need a basking area kept around 89–95°F where he can crawl out on to rocks and sit under the light. don't make it small and make sure it's good sized but also u don't want it too big because they do like to swim around. do more research and make sure u know all about him. good luck :)