This will just be a few links to good sites. 

Xpaintball       Good selection... good prices, expensive shipping.

Shop 4 Paintball     About the cheapest overall sight out there if you order alot of stuff.

PB Nation        The best place to buy paintball gear.  Cheap, and good condition.  Huge forum population.

AirGun Designs        Info and products on the best gun out there...the Automag. 

Ebay Paintball        Huge Selection, not as cheap as PBnation, but close. 

Midwest Paintball      It's the local air supply, until my scuba tanks get here!!

Arrows III                 This place is bad.  45 Seconds from our field... CO2, and paint.  Good deals too!