Try The Duchy Gunsmith. They do work on Airguns as well as shotguns and rifles. They're re-blacking a scruffy Diana 52 for me at the moment. Nice helpful guys.
The only problem with a lot of CO2 pistols (depending on what it is?) is that spare parts are pretty much impossible to get hold of...
Perhaps if you tell us what it is advice can be more focussed.