B, you might try some of the recyclers on eBay. They may not have a trailing arm on their listing, since it's not a high-demand part, but give them a call.
I have had good luck with Ideal Auto in Chicago 773-509-9999 (Alex).
also SoCal dismantles a lot of 126's.

I found new rear subframe bushings to have more effect on rear ride ht that any other thing.
Also, the 300SE has a 9.5mm rubber shim above the spring. This can be replaced with the next thicker spacer (14mm, or even 19mm) to raise the ride ht. I did the subframe bushings and the 14mm spacer and she came up a couple inches, just right.

DG