The standard o-ring that you will find at the end of a cylinder is size #15 or #015
You can buy them cheaply in bulk from suppliers such as Polymax
(Polymax have a minimum order value so either get a lot of the o-ring or get an assortment of sizes for you gear)
http://www.polymax.co.uk/o-rings/
Check the manual for your gun and it may list the sizes of the o-rings (or if you look carefully at the part numbers they may use the size code)
Alternately some suppliers give a printable sheet that you can compare the o-ring against - just take care to note whether the sheet is A4 or letter size.
In the US they use letter which is taller than A4, you need to take care printing on A4 to make sure it converts without stretching. (They often have a printed line for you to measure and confirm the size)