Where can I get stiffer than stock motor mounts?

From RPRWiki

Mugen used to make a set, but they are currently unavailable. But you can cheaply make your OEM mounts stiffer than stock by filling the hollow spaces in the rubber mounts with polyurethane (3-M polyurethane adhesive works well). Clean all of the surfaces inside the OEM motor mount carefully, apply the windshield adhesive, and let it cure for about 48 hours before reinstalling.