Hey! I get all my clients prints from mpixpro. I LOVE their suuuper fast delivery. They have tons of products to choose from and their interface for ordering is really easy. I have tried other companies, but I always come back to mpixpro (not to be confused with mpix.....that's the consumer site. also has GREAT stuff, but if you get a 'pro' account you have a lot more options).
Hope this helps and good luck!!