supposedly a 2nd gen non turbo RX-7 GXL (87-92) clutch type LSD will swap right into the miata's housing, just gotta swap over the ring gear and set backlash.
those can be found locally for anywhere between $40 and $500 depending upon your local rotary scene.
certainly a better deal than the $800 ebay no-name torsen specials.... and mazdatrix sells all the bits to rebuild the GXL diff, so if you manage to damage that one, you can fix it.... and it was designed w/a higher horsepower car to begin with... so I would assume it would last longer on your turbo'd monster too