First off, what E12 do you have? Early 530s used a single in,single out muffler, later ones used a twin pipe and 528i's used a slightly different twin pipe. Now to the best of my knowledge all 3 are NLA, but still options differ. If an early 530, single in, single out, a regular muffler shop should be able to hook you up with something. If not, while my experience with muffler shops has not been good, it would still be worth stopping by one or more and asking about options. I wouldn't get my hopes up, but the price of an inquiry is right. At worst they can do something, but be involved, I bought a car one time with a brand new exhaust system, from a well regarded local muffler shop. Well regarded by some at least. They single piped it, had about a 120 degree bend kicking up just past the half shaft kinking the single pipe, going into a muffler that droned like crazy right about 2500 RPM. Replaced it with used BMW parts, E12 muffler and E23 resonator, and it was quieter, especially at cruising speed and had a modest power and usable rev bump.
Other BMWs. E28s are close in many ways to E12s. 533 mufflers would be close, except for hangers but also NLA. 535 pipes are a bit larger, hangers are different and would have to be improvised, but are still available, for how much longer I'm not sure, and are moderately priced, you can get one from Rockauto for ~ $167 or so. But a front and rear hanger would have to be fabricated. Front, you can probably buy the parts that attach to the subframe and screw them on, I did when I had a complete E28 exhaust on my E12. The front was easy. The back I never did do right before I changed it back to an E12 exhaust, but would be somewhat more difficult. Definitely doable, but not as easy.
Then my favorite euro parts supplier, Schmiedmann's. They don't show much, but some. An euro E28 528i, both OE BMW muffler and aftermarket, which would likely have the same mounting problems, is available from them starting at ~ 250, plus shipping which from my experience hasn't been too bad. Less than I feared at worst.
https://www.schmiedmann.com/en/bmw-E28- ... 09-catn-ol And the cool thing about them is they show the clamshell design, not the tin can one, which has to be worth at least 10 bench racing HP if not more!
So there are a variety of options, albeit none great, but some decent.