I'm publishing this as a public gist so that it's googleable for anyone else looking for replacement rubber feet for their Apple IIc.
Rubber tends to perish over time and when restoring an old Apple IIc I found that the rubber feet on mine were in a bad condition. A good (perfect?) replacement seems to be 3M Bumpons, specifically part number SJ5003 (11.1mm in white). I believe these may even be the originals that were fitted on the IIc as shipped, because this story on folklore.org describes Apple using Bumpons in 1983.
Some photos:
