Any welding supply store should have the tank, regulator, and fittings, and possibly a needle valve as well. A solenoid might be trickier. Might just want to go with the pre-wired Burkert 6011 from aquariumplants.com for $47 shipped.
If money is tight, check Ebay for the regulator and needle valve. That's where your best deals will come from.
Check any homebrew supply store as well. They'll probably have cheaper regulators than welding supply.