I got this one, and I assembled it so that it was stuck to the rubber underneath the carpet doormat, so the heavy stuff is directly on the table and your typewriter is sitting on the carpet. You should be able to get two mats out of this size of this rattle dampening stuff with careful portioning.
It seems like it is going to be hard to cut into shape, but all you need do is score it with a craft knife, and then put the scoreline over the edge of a table or bench and gently bend it down until it breaks aling the score line, then slice the backing paper with your knife. Be sure and position your two parts properly first time, it really is sticky!

http://www.amazon.co.uk/dp/B012CQXK06/ref=olp_product_details?_encoding=UTF8&me=

 

I ordered a 2x3 of one of these in 1/3" thickness:  http://rugpadusa.com/premium-lock-felt-rubber-rug-pads.html

Cut it up into 6 mats.  It works well, the rubber on the back keeps it from sliding on the desk and the felt cushions and dampens sound.  It is very similar to the Jackalope typewriter pad I bought from Richard Polt.  Price worked out to about $4/pad.
 

I have three mats: a thin old rubber-backed thing that came with a typewriter, a felted one that I made using multiple strands of wool and extra-large needles, and one from the Typewriter Man. Guess which one I like bezt!

during the holidays, store sell felt placemats that are pretty thick.  you could probably double up on them for a nice 1/2" thick mat.  my dad, who loves to collect IBM Selectrics, actually has a wool mat that's at least an 1" thick.  i would love to find another one of those for my typewriters.

Beng honest though Uwe, that isn't a felt mat! IT's $150-worth of wnderful but very exensive thick felt. Bit of an outlay...