There are lots of places online. The thing is, it's usually best to jump around and find who is having a sell right now. Dubais are a weird insect in the sense that sometimes they seem to be in a shortage, and other times a company or breeder will have too many and be selling them cheap! I'd just look around google and see if you can find a place selling them for a good price.
After a few websites, you'll get a feel for their current pricing. I and a lot of others I feel don't really actively buy dubias. At least I try to only have to buy them once, and enough to start my own colony.
-Brandon