Do you know what type the breaker is? If it is the old Zinsco style I think your going to be out of luck. I don't think they even made two pole GFI's in this type. If the breaker looks similar to a GE or Siemens breaker then a Cutler-Hammer QBGF2040 may be what your looking for. The Zinsco line became Sylvania for a while then Challenger then Westinghouse and is now Cutler-Hammer. If you can find any more info about the panel or what type the other breakers are in the panel post it and I might be able to provide more help.
From reading the reply's to this post and other posts it sounds like many of you have a hard time finding old breakers. I am in the San Jose, CA area and we have several suppliers that specialize in old breakers. If they don't have what your looking for on their shelves they can usually get it for you the next day. For anyone that needs a hard to find breaker try:
POWER BREAKER (408)453-3757
BAY BREAKERS (408)998-2980