One option is to look for a "totalled" bike where the frame is not materially damage (road rash is OK, but bent frame members or cracked welded joints are not). You are looking for totaled bikes where the engine or tranny was significantly damaged and the insurance company totaled the bike due to the cost of repairing the engine/tranny. Or a bike that was in an accident with significant cosmetic damage but no material damage to the frame.
My bike was hit by a cager from the left rear. The repair cost was over $14K which was paid for by insurance but only because the bike was a '12 UC. An older bike would probably have been totaled.
Cheers,
TQ