Help!!! My 2240 broke

[funnyrunner]

While taking the CO2 cartridge out today, a small yellowish gasket came out with it. It obviously goes where the neck of the cartridge plugs in but how can I replace it? Is it possible to get a rubber replacement gasket from Home Depot and use it instead?
I tried placing it on the end of the cartridge and pushing it back on but when I tightened the cartridge down,all the CO2 just leaked out.
Any suggestions would be greatly appreciated.

[ped]

you'll need to strip the gun to get the valve out then refit a new seal

[funnyrunner]

Where can I get a new gasket or do I need a new valve assembly?