It would be useful to know where you're located. In the USA, you might try these vendors:
http://www.pbparts.com (parts and repair work)
http://www.dttservice.com (parts and repair work)
http://www.powerbookresq.com (parts and repair work)
http://www.tekserve.com (parts and repair work)
http://www.powerbookmedic.com (parts, repair work and instructional videos)
http://www.welovemacs.com (parts and used equipment)
http://www.ifixit.com (parts and excellent, illustrated online take-apart instructions)
Prices for the same parts vary WIDELY, so check around. Also, parts as small as the rubbr feet you're looking for may not be listed on the websites, so calling the toll-free numbers you'll find on most of the sites may be a good idea.
If not successful in locating the Apple parts, you may want to pick up some self-adhesive furniture feet at your local hardware store. They won't fit perfectly or last very long (heat breaks down the adhesive on them), but they may cost only a tiny fraction of what you'll have to pay for Apple feet.