One option I haven't seen mentioned is a used axleshaft. I had the boot split on my old 123 after a loose fuel line soaked it with diesel - rubber and petroleum products don't miss.
I purchased a used axle from AAA Small Car World over in Ft. Worth. Don't recall the exact price, but it was about 50 bucks. That was long enough ago that I cannot even remember the year I did the work, and it's still in there, still working. I'm thinking 1995 or 1996. In fact, my friend that purchased the car is staying at my place for a bit, maybe I'll crawl under the old 300D and see how it's doing. (Not likely in this Texas heat!)
I understand the boot crimp tool makes a good doorstop.