What is the best site to buy backgrounds?

Denny's has a lot of nice backdrops and floors too.

I often use curtains from Walmart or Bed Bath and beyond for a cheap backdrop for 3/4 shots.

Maybe you are looking for Digital backgrounds, in which case disregard my advice.

several digital places for backgrounds , deviant art has some free but most have rules to use them , so paid prob istock.com , dreamstime.com or shutterstock.com , I personally also use dollarphotclub.com , if you are going with a membership and will pay a higher price prob istock.com