OK, I could not resist looking around. These folks seem to have collected battery covers off Junk Cameras.
Battery compartment covers are tiny and easy to misplace- so it might not be a bad idea to grab a spare if you're worried about it. All the parts we sell are pulled directly from original equipment- they are not reproductions. Parts are checked for function, but they may have small cosmetic...
bad news is that they don't show any in stock for an FM-10, but it might not hurt to ask, they are showing prices in CDN$ so the USD price would be about 75% of that shown.