Take a look at the American Vintage Airguns Forum.
There is a heavy lean there towards old target guns.
Many of the old target guns are not well known enough to be sought after, thus, reasonably inexpensive, for what they are capable of.
An FWB 300, unfortunately, is a recognized "brand name" bit of work that usually draws top dollar.
You can get lucky, though, if you are not too concerned with originality or collectability. There were lots used as club guns or junior guns, and many of them were mch modified over the years. They don't shoot any worse for it, but may look cobbled together (stocks, stickers, etc.). A handy guy could build a pretty decent looking stock for not much money. Or rebuild a hacked up one.
Cheers
Trev