Contact everything airbrush, they sell packs of o rings, they'll sort you out, not open until the new year. At a guess use some PTFE tape as a quick fix.
http://www.everythingairbrush.com/ You'll have to phone them they're not on the site.
Other than that any o ring will do but unfortunately I don't know the size, doesn't it say in the manual ?
Also, if you're using water based paints ALL the time rubber based O rings are fine. If you're using Solvent based paints you need to get Teflon ones.