British Cycle Supply Co. has them for $4 list price. I've used BCS often over the years and they have a local number now that goes to Nova Scotia and a shipping warehouse in NJ.
I need to build up the height of two of the buffers on my seat to take up some clearance it's developed and plan to glue some cut-to-thickness shock grommets onto the pan to accomplish that. If you don't care about complete correctness, I think it will work just fine. My buffers were glued down as well as being fastened into the holes in the pan but with some cleaning and good gluing I don't think the lack of the little nubs is going to matter. If I do lose one, I've got lots of old grommets.