Use 720K PC floppies if you can't find 800K Mac floppies -- they're actually identical media. Only the formatting differs. In an emergency, you can get by with a virgin (not just empty, but actually demagnetized) 1.44MB floppy. Cover its media identifier hole with a piece of tape, and you're off to the races. This trick is not completely reliable, and so should be used with that in mind.

BTW, the oldest OS that Apple provides directly is system 6. If you need something older than that, you'll have to go elsewhere.