I put my C02 tank together from parts on the Internet for under $150. I spent around $80 for a 10 lb. aluminum C02 tank, $50 or so for the regulator, and the balance towards a quick connect & misc. Make sure the tank you get is aluminum, some tanks you'll find are steel & much heavier.
Here's a google search showing lots of the right kind of tank... https://www.google.com/search?q=10+l....1.LIQoVxvZjk4
My regulator looks like this...
which is sold at The Original SOURCE HyperFLO Fixed Regulator
. I didn't buy mine from them, I just ended up with one that looked like theirs after rounding up the parts to make my own.
The rest of it is just putting together an air hose with the right fittings on both ends. All in all, it's an easy project.