Every now and then, you can still get a good buy on ebay.
Described simply as:
5 1/4 Full height floppy disk drives
this lot:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4193&item=51…
is not one, not two, but three MPI B51 floppy drives. Ohio
Scientific used this model in all of their "MF" (mini-floppy)
offerings. If you don't want to bother following the link,
I got the lot for $10 plus shipping. One of the drives has a
gray faceplate, which I've never seen before. I've had a hard
time figuring out what other manufacturers used B51s. Atari
used the mechanism in some of their 810s but with their own
custom electronics. Before any Atari enthusiasts scream, I
have not and will not disassemble any working 810s for parts.