Industrial hose & gasket, HVAC suppliers usually have adhesive backed dense foam in different thickness for a good price. Buy a sheet, cut it to whatever shape you need and stick it on.
Minicell Foam is the best.... You can sand the edges smooth and the Contact Cement Holds it great.. I have made about 3 seats with it... http://www.campmor.com/riverside-minicell-outfitting-foam-adhesive-backing.shtml?source=CI&ci_sku=58631WC&ci_gpa=pla&ci_kw={keyword}
Got my foam padding from a local craft store. About $1 for a square foot piece. Got a couple different thicknesses. 2-6mm. And yeah, they have pink ones too. For ages 3 and up :P Product link: http://www.darice.com/ecom/ProductDetails.aspx?it=1144-12&oid=6089 Not sure how it will hold up to the friction of my ass as I havent use it yet. Seems like pretty tough stuff though
We borrowed a tail section, that had a surf board kick pad. http://images.search.yahoo.com/sear...age&fr=chr-greentree_ff&va=surfboard+kick+pad Tyler really liked how it gripped his leathers and it has a kick up that was put towards the back. Ours was cut up into a few sections on the SBK tail.
I've had good results with 1/2" HQ neoprene foam from www.foambymail.com Ordered a half sheet of 1/2" x 80" x 24" for $32.99 which will make around 10 pads for my bike. Cuts easily and the edges can be shaped with sandpaper. Used 3M Super 77 spray adhesive to attach. Held up well for one season of trackdays, and seems it will be useable for years.