not cheap, but "one click" ordering. part number is there, too, so you can take that to your local dealer.
a less expensive choice would be the radio pocket from a 99-00 miata. no door on it, like the mazdaspeed, but at $15 its a LOT cheaper. (fits the same too)
i have the MSP support bar and the miata pocket in my car right now - i know for a fact they are interchangeable. any mazda dealer can get it for you - part number B02A-66-9M0A