personally i like
Greenleaf Aquariums as a sort of one stop shop to get every thing and be done with it. admittedly there not the cheapest but thay have good regulators and every thing else also good customer service and fast shipping.
Rex Grig also has a bunch of stuff for sale dealing with CO2. haven't dealt with him personally but have heard real good thing about his regulators and his customer service.
if you only looking for the regulator i would check out
Sumo Regulators. this is where i got mine from and the thing is rock solid. they usually send along tubing and a check valve with there rigs too which is nice.
and if you the DIY type of person you could always build your own regulator from parts you. it tends to be cheaper and you can usally find good deal on regulators and needle/metering valves on ebay or other places.
btw what is the wattage you have over your tank medium lights don't say much