The mac-midi-adapters archive contains two different versions of a self powered adapter that should be usable with any Macintosh.
The basic unit has two outs and one in, with a single status lamp that shows if the port is powered (on macs with a powermanager such as the powermacs, etc, this is very useful). This unit is the one I use when 'on the road'.
The deluxe unit has a THRU switch to it can be used without tying up a port on the machine, and TX and RX lights, of much help when trouble-shooting MIDI problems. It is more oriented towards desktop use, but of course they all work on any mac with MINI-DIN-8 serial ports.
USB to MIDI Interface Roundup