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I have the AK-104B, which is both modules together. Or if you scroll down, you can purchase the modules separately. The button used should be the same as the OEM button.
I don't think swapping the BRZ push button in will be easy. The OEM setup requires a new steering column for the lock out, as well as a different ECU for the key/transponder. I think aftermarket is going to be the cheapest route!