If you have a "special edition" sort of car, like a gullwing or Grosser, you will be able to have parts, since these are so rare and you can charge arms, legs and first born from the buyer. With multiple hundreds of thousands of R107 made, we are not special, and the w114/115 is even less so.
Red, don't worry, you are special to someone, I'm sure...
eBay & eBay.de are always there, as are places like SLS.de, Adsitco or Niemoller, aside from the other sources mentioned above.
Classic Center... What a catch 22. They support the owners who support them, and those owners typically have cars that they don't mind spending $$ on, which depending on the chassis, can be a very small subset. Sure there are more 114/115 out there than Gullwings or Grossers, but darnit, like many of us, their owners are typically quite frugal. The cost of limited run production for some parts is high, and the return not likely there in most cases.
I believe it has less to do with whether a car is a special edition of sorts, but you hit the hail on the nead with numbers made. With so many 114/115s made, they really became common cars, and were treated as such. By now many are on to their 4th or more owners, and with their values as low as they are, the typical owner is not keen to pay Classic Center prices for their parts, some
of which are arguably better than what the aftermarket supplies.
When the majority of owners stop considering their car a classic and won't go to Classic Center for parts, it doesn't really remain a sound business decision for them to stay in the supply business, I suppose.
It is the old used classic proposition; "this hooptie is only worth $3,500 (or less), so why should I pay X for the proper parts and maintenance to keep it pristine".
I personally don't care what the market value is of my cars, but I do take pride in keeping them up. Sometimes that means paying the freight (literally--I am in Hawaii) for proper parts and maintenance...