Without wishing to sound facetious, the search function on the forum is brilliant. I typed in "buggy foam" and found these threads so far on oOple:
http://www.oople.com/forums/showthre...ght=buggy+foam
http://www.oople.com/forums/showthre...ght=buggy+foam
http://www.oople.com/forums/showthre...ght=buggy+foam
There is also the following thread on the COBRA forum:
http://www.swmcrc.co.uk/forum/index.php?topic=290.0
But just in case.....
follow this link:
http://www.rc-direct.co.uk/-c-39_198_205.html (disclaimer: other companies sell them but this is where I got mine) buy two of the same shore rating donuts (the higher the number, the harder the foam is and the longer it will last but won't be as grippy). Cut them to the width of you wheels (one donut will do two wheels). Prepare the wheel by taking the ribs off if you want to. Stick them on (contact adhesive, double sided tape etc.). True them down to what ever size you want.
The club has a truer you can borrow. Make sure you round the edges off well as if you don't you'll get grip roll.