Why not take advantage of recent developments in Phage Display of
repertoires of Ab-fragments? Even if your protein is higly conserved,
you can select either scFv or Fab (depending on the library) against
it if the library is 'naive', i.e. the diversity is created synthetically.
The problem is getting the library :)
Look for papers by Greg Winter on Medline or send me a mail and I'll
get you some refs/adresses.