How cheap are you looking for? CO2
tanks aren't exactly cheap.
For a 15g, you would be fine with a 5lb tank.
I ended up buying mine from http://www.aquariumplants.com
because it was cheaper than what they are sold for locally. But, you have to find a place to fill your tank instead of swapping them in (since you bought a shiny new tank) and for me that was hard to find.