Think carefully about the choice of air cylinder
Going back to the original question all modern paintball guns are designed and can run on compressed air
Some old school systems work best on co2, and in some cases are best on liquid co2 (but I digress)
With buying an air cylinder you have about 3 ranges of cost:
1) about £30 - £40
New 3000psi steel/aluminium cylinder - good for 5 years life
It can be retested for unlimited life but they are cheap enough to replace
2)about £80 for a second hand 4500psi fibre cylinder
But as mentioned be wary of the life, and possibly condition
Estimate the life cost of a fibre cylinder at £150 plus 2 x £25 retests = life cost £200, average of around £13 per year.
You need to take that into account when buying second hand. If your purchase price plus any remaining tests, divided by the remaining years to its expiry are over £13 then financially it's not worthwhile - you're just subsidizing the sellers upgrades
3) about £150 for a new cylinder, but take into account that it is a commitment over 15 years to get it retested - life cost £200
The above figures assume you can get tests arranged at £25 with no additional postage costs
Financially the 3000psi cylinder is best value for money. If you need more capacity you can buy 5 cylinders for the price of one fibre
The majority of sites only fill to 3000psi, but events may have 4500psi