I have purchased equipment at warehouse clubs like Sam's, BJ's, CostCo. The main things to look for:
snorkel and mask that have baffles that allow you to expell water without taking the mask off
large fins, and preferrable adjustable
a mesh carrying bag is nice (breathable, so that the equipment dries
I even bought a wetsuit at Sam's. I was shocked at how check it was ($30). Works great so far!