Jack, over here i buy them directly from the Schaller importer. Try doing a search for Schaller in the US or if that fails i think Stew Macs and/or Allparts sell spare's.
Jem
Another idea is i think AdmiralB uses Mighty Mite ones and says they're, within reason, compatible and probably a cheaper alternative. Might be worth dropping him a line to clarify as i may be mistaken...well i was when i got married !