I saw a Carol Kaye clinic and she still uses just a piece of foam stuck in there. Looks like something you could get at a hardware or craft store.
Actually Carol uses felt taped over the strings with a pick. If you play finger style then use foam under the strings near the bridge. If you go to her website there is a post on this topic under her forums link. Additionally, if you have a nitrocellulose finish be careful of the type of foam used as some cause a chemical reaction that will melt the finish.