Quoted:
5 gallons? Are you trying to dispense beer with a portable system?
Here's the problems, in no particular order:
1) A single 12g or 16g cylinder is not enough gas to dispense the whole thing. The 1/4cuft only applies to standard temp. and pressure. Beer is stored in the keg at around 10-15 psi and 35-45*F, depending upon style. You'll get even less volume out of a cylinder under those conditions than calculated.
2) Those cylinders generally contain air tool oil. It's left over from the manufacturing process and is actually beneficial to paintball guns, air pistols and tools that they may be used in. For food or beverage uses it puts a sheen on your beer, destroys head retention and tastes kinda funky.
3) You need to regulate the pressure anyhow unless you just really like foamy beer.
You're looking for a product like this:
http://www.midwestsupplies.com/products/ProdByID.aspx?ProdID=7980
But without paying $135, right?
Trust me here, it has to cost that much. The regulator is the expensive component.
You can grab a 3.5oz or (if you can find them) 7oz. cylinder for paintball use. Larger than that and they need periodic hydro every 5 years which means that it's really only good for that long before you should just buy another one. That's $25 for the tank. The regulator is going to be at least $50 plus some fittings and an air source adapter (pin valve depressor) will run you another $10-20. You need a keg coupler, too. For Cornies, $5. For Sankey, that's $30+. If you get everything right the first time, you're looking at around $100 for the setup and it will not be as small and convenient as you think it will be.
If you really find the need for this product (i.e. don't feel like just disconnecting your tank and reg and taking it with you), you're better off just buying the commercial one.
One last tip - if you do get a paintball tank, feel free to get it filled anywhere. Paintball stores just buy their bulk CO2 for local gas distributors, which serve mostly soda machines. It's all food-grade stuff.
The tank itself, however, should be Cr-Mo. Do not get a steel tank (they're old, but you can still find them - notably Brass Eagle brand 4oz. tanks) Catalina makes a 3.5oz Cr-Mo tank that you can buy most places for about $25. I recommend wevopaintball.com and tell him SCP sent you.