My guess is that it's not generally available as a user-installed item, though you probably can buy it from Mac parts sellers.
"This AirPort Extreme Card is not compatible with the Mac Pro or the Power Mac G5 Dual and Power Mac G5 Quad computers introduced in October 2005. To add an AirPort Extreme card to one of these systems, please contact an Apple Retail Store or other Authorized Apple Service Provider."
I did see an Airport / Bluetooth combo kit on clearance at a CompUSA but I didn't bother to get it, and it was for the Oct '05 Power Macs, it may or may not have worked with the Mac Pro.
I personally don't expect to use either much but I'll probably have them installed just in case.