Mouse feet are apparently made of teflon aka Polytetrafluoroethylene.
I was resigned to buying a broken G9 on eBay and salvaging its feet, but apparently, much to my surprise, it is possible to buy aftermarket mouse feet aka slider pads.
Here are two from HyperGlide:
http://www.slicksurf.com/ also sells a variety of replacement mouse feet, both custom fit to particular mice and generic one-size-fits-all. However, they don't have anything for the G9, and I saw one user review that was negative:
Please don't recommend Slicksurf to me. I've tried those and I thought they were just pieces of plastic that were not slick at all. Not to mention they were not cut precisely like other feet I've bought. Furthermore, because of their hardness, I felt like I was grinding into my mousepad everytime I used my mouse with those things on (Archetype rough mousepad).
The reason I listed the thickness, above, is that I need to be careful that I'm not getting flimsy, ultra-thin teflon tape (I've tried this before, and it sucks), but actual teflon plastic feet.
I also found a video of somebody applying the HyperGlide custom mouse feet, and they indeed look like replacement slider pads:
http://www.youtube.com/watch?v=9aJt9Ei1HXo
Also check eBay as I found a number of HK vendors selling good thick teflon G9 feet for cheap there too.