It's a pretty unusual plug, possibly something Apple custom designed. It's not complicated though, so if it's not custom, you may be able to find it. It's like a stereo phone plug with a shield around it.
If you can't find it (I've never seen one for sale, but haven't looked too hard either), you can get away with just using a stereo phone plug and ignoring the shield, as apparently the shield is only used for the LED, which is not really a necessity. I don't know which size the 867 MHz PB uses, but I'm assuming it's 2.5 mm like my 1.33 GHz 12" PB. The ring (bottom section) on the plug is the positive supply voltage (24 V), the middle section is the negative/ground connection, and the tip is not connected. If you can find a connector with the outer shroud on it, connect a relatively large resistor between the negative/ground connection and the shroud (something like 500K-1000K).
Try
www.digikey.com or
www.mouser.com for the plug itself. 2.5 mm is not a terribly common size for a 3 conductor phone plug, but they are available. If you have a nearby electronic parts shop, they will probably have it.
DISCLAIMER: I've never done this myself, and if it causes damage to your PowerBook, I'm not responsible. You would be wise to measure your good adapter to verify that the information I just gave you is correct. I'm assuming that you really do know what you're doing, and will do a good job of wiring the new plug on so that it doesn't short out or apply a harmful voltage to the wrong input terminal in your PowerBook.
