www.aquabuys.com
get this
http://www.aquabuys.com/miva/merchan...roduct_Count=0
it is about as cheap as you will find a package almost complete (less the tank) you will find.
You should be able to get a 2.5lb or 5lb tank from a local weld supply shop or a home brewing store.
Some fire extinguisher places will have them also.
If not there are some online home brewing sites (
www.thebeveragefactory.com might be one)
that sell them for good prices.
5lb should cost you about $65 or so. Don't get one if it is over $85 I would say.
I say 2.5lb is an option for you since you have a small tank. I used one on a 29 gallon and got 5 months out of it all the time, so it is enough if you can get one for cheap. Some will say it is too small, but only if having to fill it every 6 months is too much for you, is it too small for you.