Yes, you can use any fixture for the light. The only thing "special" about the ones at the pet store is that it has a hood to direct the light downward, but as long as you follow directions for how close the light needs to be, it doesn't really matter.
I actually recently used a cheap under counter mountable fixture from Walmart and rewired it to fit into a hooded aquarium light I had that stopped working. Was a PITA, but worked out great.
Yeah, they are super cheap at Walmart just make sure you take the plastic cover off of the fixture so nothing will block the UVB light.
Yes, Unless your Walmart has a ReptiSun 10.0 fluorescent tubes (mine doesn't) you will want to get a UVB 10.0 from a pet store or online just make sure if it's a T-8 Fixture you get a T-8 Bulb or T-5 fixture get a T-5 Bulb with the same inches. I use a ZooMed Repti Sun 10.0 36" T-8.
T-8 and T-5's are fluorescent tube sizes. It's like the diameter or where the sockets plug into the tubes. If you can't tell by looking at them (it's easy T-8's are a bigger diameter than the T-5's) It should say what type of fixtures they are on the box. The most common sizes are T-5s, T-8s and T-12s you want a T-5 or T-8.