Here is a store that sells 5 different brands of panniers:

http://bicyclehabitat.com/product-li...panniers-1163/

Considering the relatively limited market, I think five different brands sounds like a lot. Now whether they will have all makes and models in stock is another story. Personally, I think it's unlikely that you will find a shop that routinely keeps a "large selection" in stock, but I could be wrong.