Stinpie,
I had a good experience with Super Soundproofing.com which sells, among other things
Super Soundproofing Mat. A one linear foot piece (it comes from a 48" wide roll, which would be more than enough, I would think), of even their thickest 2" mat would only cost you $31.50 plus shipping. And I'm not sure you would need a mat that thick.
It's not hard to work with, either. Place a straight edge where you want to make your cut, draw a box cutter, razor blade, or any other sharp implement, lightly along the straight edge a number of times gently pushing a little deeper with each pass, until you're down into the mat a quarter to a half inch (depending on the thickness of the mat), then use a good pair of scissors to carefully cut the rest of the way through.
If you cut the mat to the same size and shape as the speaker, you could just set the speaker directly on the mat without spikes or feet. I have their 1/2" mat between my M60s and some stands I made for them.
I dealt with them on the phone and they were very helpful, and didn't make fun of me ordering only one linear foot.
