There are constant air adapters, but they are pretty rare. Try PM'ing bacci or finding "sugarstump" on ebay and sending him a message. They have pretty big vaults of old school parts. Another option is just to use a dummy 12 gram and run it to a tank, but then you need to figure out how to mount the tank. Small tanks in/under Crosman stocks seem to be a popular choice.
You could also just make a plug out of aluminum and thread it, if you can figure out the right sized die to use.