Well, Beeman doesn't exist anymore in the same way they did back when you bought the rifle.
Even back then, they didn't manufacture anything, they pretty much just imported airguns and rebranded them. Even in their heyday, I wasn't too impressed with their repair service (or customer service for that matter).
Like Jay said in his reply, you can look in the GTA Mall link in the info bar (under the picture of the gun up above). Additionally, there are several folks here who can replace a spring for you, maybe one of them is even local, so you don't have to ship! Where do you live?