Just visit all your local airgun shops on a regular basis with a fat wallet. Some gun will jump out at you and say "Take me home!"
Well, thats what happens to me.
Honestly I wouldn't know where to start collecting these days. A few years ago, when I started collecting seriously, you could pick up as new BSFs for £40. I did! Now, anything with age or character has an expensive price ticket attached.
ATB
Ian