i know this is old, but wanted to throw this in...
i converted a 14" acoustic shell recently, an was having trouble finding rim rubber for that size. not sure where you're located, but i was able to find a third-party ring manufacturer out of Arizona/New Mexico. they were kind of pricey, i thought, but not absurdly so. i think i paid about $15 + Shipping for the 14" model. they are handmade and very good quality - a little thicker than the stock rim covers provided by Alesis. The company calls them 'Rim Condoms.' not sure if other users here have used them, or what their experience has been, but mine is great.
i realize that this may be too little, too late, but wanted to mention it, in case others find this thread in a search. if you're doing just a few rims, or smaller rims, at a time, the cost seems fair, for the most part. however, it also dawned on me that you could probably make the same rim covers at home for little money, so long as you know aht you're doing.