kegging 101
needed parts:
1 co2 tank.
1 regulator. dual regulator recommended so you can read tank and keg pressure
hoses.
connectors.
at least one cornelius keg. (aka soda keg, corny, 5 gal keg)
a total system will probably set you back about 170 +shipping. You can find parts elsewhere, though the time you spend searching isn't worth it IMO.
tank ideas: I understand that large fire extinguishers, the old kind, use 5 and 10 pound co2 tanks...However, tanks must be sqeeze tested every 5 years and may eat up any savings you might have made. PLUS you will still need the valve fitting and everything else.
regulators and tanks can be found on ebay...
check your want ads, join a local homebrew club and see if you can find someone unloading old equipment.
Anyways, I will still argue that the best bet is to lay out the cash and buy a system. You'll love kegging and your only challenge will be to brew fast enough to keep up with demand.
Its a good idea to have at least 3 cornies. I have 10

.
There are places where you can buy them cheaper than at a local brew shop.
ebay: usually a safe bet, but watch out for shipping and 'handling' charges... you'll probably be looking at abuot $20/keg
independent resalers: these guys are out there.... Green Bay Rackers used to sell them for 10 or 12$....not sure if they still are.
PM me if you want an email addy of a guy in Wi selling supposedly 1000's of them. I haven't bought from him, but he seems alright from his emails. probably 12-15$ or so.
Kegs sometimes are dirty, need cleaning, scrubbing even, and might be sort of sad looking. You should consider new gaskets for your kegs, and there are cleaners out there that are recommended. Bleach is not a good idea because it is reactive with Stainless Steel. That 35$ keg at your homebrewshop has been refurbed and is garuanteed to hold not leak.