Whew! Not an easy question.
Hard to comment without knowing exactly what sort of regulator and cylinder valve you have. It's probably DIN (used in most of the civilized world, North America and Japan notwithstanding) but it might also be the strange external coarse thread connector used on some of the small paintball and soda fountain cylinders.
On a good day, it would be possible for welding supplier to put an adaptor together using a DIN 477 (CO2) nut on a CGA 320 1/4" nipple screwed into the bottom of a new CGA 320 cylinder valve. The DIN nut and the new CGA valve would quite likely be special order items.
If the inlet nipple on your regulator can be unscrewed, it should be possible to get a CGA 320 regulator nut from a welding supplier to replace the DIN nut and buy a new CGA cylinder. This appears to be what one manufacturer already does, judging by the pictures posted on one of the "one o-ring, or two?" threads posted a while back.
Check with paintball suppliers. They seem to carry a number of adaptors for weird equipment, so you might get lucky.
A machinist can order the parts and machine the hard to get stuff, but at a certain point it will be cheaper to buy a new cylinder and regulator.