McIntosh C28 transistor subs

Depends!!  Basic transistors yes, but some makers use all sorts of esoteric specd parts.

I would call around to some other Mac service centers and see if they have any that they can sell you.

Not always. The biggest problem you run into is the surrounding circuits are tuned for the spec'd transistor (or vice versa) and a substitute that may seem the same may not work in that circuit. Trust your tech on this one -- he doesn't want to work on this twice.

Of course aftermarket transistors will work. Transistors are color coded so they can be identified. Take close up pics of them, look up the color codes and then call a couple of parts houses and get the tight tolerance versions of them and have your tech install them. 

Try the audiokarma forum, they have threads dedicated to McIntosh and technician issues like this.