I would call around to some other Mac service centers and see if they have any that they can sell you.
McIntosh C28 transistor subs
My C28 needs two transistors that are on backorder from McIntosh for months now. Can't a high quality brand transistor with similar (or better) specs do the trick? I do not know the part number of the transistors in question, but my repairman wants to await the arrival of the McIntosh-supplied parts. Your input is welcomed!