These guys have a huge assortment of aquarium related products:

• Food
• Water Treatement
• Filtration
• Hardware
• Shipping Supplies

Plus, they are probably one of the most helpful and nice vendors I've ever dealt with.  If you have a question, give them a call, they are super nice, will answer any questions that you have, and typically my orders show up on my doorstep the next day.

I highly recommend Jehmco for any breeder or serious hobbiest.

These guys have a wide selection and good prices on pumps of all sorts.

Also a good selection and prices

I get most of my PVC from these guys.  They've got good prices and a huge selection.

Low torque ball valves and a bunch of other types of ball valves with unions too.

An incredible selection of all types and sizes of PVC valves and unions.

Really cool aquarium equipment and filters.