Without an adapter, the CO2 regulator accepts 88 gram CO2 cartridges available from WalMart and paintball stores, at approximately $5 each. These cartridges provide around 1.5 hours of engraving  Link to 88 gram cartridges (also check your local WalMart  paintball section)

The regulator with the 25g adapter  will accept 25 gram CO2 cartridges, which are available from bicycle shops, at approximately $6 each. These cartridges provide about 20 minutes of engraving.   Link to a 25 gram cartridge

The regulator with the 567g adapter  will accept the larger paintball CO2 cylinders such as the 20 ounce, 567gram cylinder. These cartridges provide around 8 to 10 hours of engraving. These are refillable at a local paintball store for $3 to $4  Link to a 567gram cylinder

CO2 regulator for CO2 cartridges and cylinders $142
25g CO2 adapter for fitting threaded neck 25 gram CO2 cartridges $8
567g CO2 adapter for fitting threaded neck 567gram (20oz) paintball cylinders $38