I agree with aeajr. If you need more than just a wing or tail it's better in the long run to buy the entire plane.

As an example, I just finished building a Slow Stick. I bought the Slow Stick Slope Glider (NPS) version. I can find it for $18.90 on various web sites. Just a new wing and tail feathers alone will cost $18.00. For $0.90 I can get a new fuselage (I know that's $10 at my LHS), mounting hardware (a couple of dollars), decals ($3 or $4), control rods (maybe a buck), glue, and other things I'm sure I forgot that would probably total about another $20.00 or so.

Just buy a new plane and be done with it.