There are many fewer reptile keepers compared to people that keep fish, so there are many fewer vendors for things like heat lamps and uvb bulbs. When it comes to turtles as opposed to reptiles, that number gets even smaller.
That means that a well supplied Petsmart or Petco has most of what you could find anywhere. Steve mentioned two reputable food brands. I primarily use Omega One and supplement with Mazuri wheat pellets and Hikari turtle sticks, as well as fresh veggies.
For uvb and heat, I'd stay away from store brands, but the national brands you'll find at the big chains are almost all you'll find. I use a MegaRay for uvb/heat, which has a good reputation online.
For basking platforms, nothing commercial is worth anything for an adult turtle. You need to build your own.
Tobi a RES born in 2012
1 dog, 1 teenager, 3 aquariums filled with fish, snails, shrimp and a bit of algae