I know of two types of places to check, but the prices aren't much lower at each. Depends upon how many your getting and how far you have to travel. Radio Shack is close for me, so it has been the cheapest choice.
1st place is to check Smart Home at
http://www.smarthome.com
2nd place is to check with your local electrical distributor that tailors to the electrician. When I finished my basement I got all of my lighting, outlets, GFI's, switches, cable, Category 5 connectors, etc at this place. Their prices were cheaper than Lowes or Home Depot and I had a lot more options to choose from.
Ross