Try your local harware store
I have the same problem replacing them on my H33. While in the hardware store last week, I noticed a bin in the hardware section called "nylon snap bushings". These look like the exact replacements (except my old ones are white). Measure the diameter of the hole you are filling, and buy a few for fit. They sell for about 25 cents each I believe. Be aware that the stores don't seem to stock in high quantities, so don't wait too long before buying them if you are replacing all.Ken Palmer, S/V Liberty