Locally I don't know where. But I bought 1/4" and 1/8" closed cell ensolite foam padding that's commonly used for sound damping for car audio instalations. The 1/8" came from RAAM mat (bought second hand on craigslist) and the 1/4" came from a specialty sound abatement company in Houston.
Ensolite is also used for yoga pads and backpacker's sleeping pads. The Miata cockpit is so small, you might just buy two 1/2" camping pads from a sporting goods store and cut and install.