aquatic ecosystems(http://www.aquaticeco.com/)
though not specifically a hydro store, they carry a full hydro line, plus lots of other useful things.

usplastics.com is not a hydro store, but has good prices on plastic and acrylic components.

I have ordered from these guys and I am happy with my nutrient solution. I use the hydroponics special with CaNO3 and MgSO4 added to it. There prices are pretty good.
http://www.hydro-gardens.com/all.htm

http://i140.photobucket.com/albums/r30/rotti4130/DSCN2977.jpg
my growth

new add htgsupply now has grow bright fan and filter combo at a great rate i just recently brought another 600 electronic ballast light system although thier on back order they are giving away argomax hps bulb if you order before the 20th

stealthhydroponics.com
i also brought two automatic ph measure and adjust unit from stealth hydroponics set it and for get it but each unit has to be set for either or ph up /ph dn me i went over board by buying a second unit for the down but hey better safe then sorry my resvoir are located in my garage and darn outside being thier

Automatic pH Controller Reservoir Kit 199.00

also they have a Multi-Parameter Water Quality Monitor
pH, EC, PPM, CDF, Celsius temperature and Farenheit temperature. Includes a 110V/120v power supply transformer.
at a cost of 149.00

so if your like me i like to do as little work as possible at a very cheap price

BUMP!
Just cause I think a good source list deserves to stay near the top.

bump!
'cause I was lookin'!

My 2 cents on cheap hydroponic shopping is .... none other than Ebay! I have yet to find one site cheaper, when you count in the shipping, than Ebay. In specific I recommend the shop called "thehydrosource." Best prices ever!

You will be burned on EBay.......maybe not today or tomorrow, but you will be burned.

http://horticulturesource.com/

Good prices, huge inventory