I went to the Apple Store in Lynnwood, WA and had both Airport and BT installed in my Mac Pro, total cost was $74 including tax and installation. For some reason, the store personnel were insistent on installing the modules for me. I guess they have had problems with self-installations by end-users. The modules get popped onto the motherboard and there are preinstalled wires that need to be connected to those modules.
Interestingly, I bought my Mac Pro from Powermax (see above posting) but they claimed not to have the Airport or Bluetooth modules yet from Apple.