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.  

Wait for the part from McIntosh.

If "any" transistor will do, I would expect Mc would have filled your order by now.  Weather be tightly spec'd or proprietary there is a reason for the wait.

I personally would look up the transistor on Mouser, Digi Key, Ebay?... and go for it,  But I do the work myself and take the gamble.

Don't tell the tech to go look for another place to get the transistor.  Unless you are willing to pay him his hourly to do it.  If you get the part and he installs it.  He's not going to warranty the part.  You take the gamble, No guaranties.  With the OEM part, there's better chances everyone will be happy.

Color Coded?

at digikey all you have to do is to punch in parameters of your original transistor and you'll get a variety of ones with different fittings and shapes.

In case with transistors, unlike tubes their parameters may differ to a larger tolerance.