I just bought a new engine mount from custom performance engineering. They also have a rear trans mount available. They come with stiffer bushing as well which is nice since the stock ones suck and have way too much play. You can also try turbine tech, their are two different rear diff mounts on the speed6 so i'm not sure which one your looking for since your didn't specify. Theres a top and bottom mount. Turbine tech has the lower rear diff mount and they also offer a front engine mount. I bought the front engine mount from turbine tech as well as bottom rear diff mount from them. I bought the top rea diff mound and wishbone engine mount from custom performance engineering "cp-e.com" I've these goodies for a couple weeks but havent been able to put them in yet or id tell you what kind of a difference they made. turbine tech has some good info though, check them out
Hope this helps...