Well, I wrote to the seller, he made me an offer and I accepted it. So now I have a Roland MPU-IMC (later revision with the IRQ jumper) with the breakout box and manual. If you are interested in the package, please look at my wishlist and feel free to make me an offer (cash alternative is also welcome).
Your best shot is finding IBM PS/2 Model 57 / 77 systems. They came as multimedia editions with CD-ROM (SCSI) and CT5320/CT5330 on board.
I found both in SC last year. Sold both CT5320/30 within days. I found them whilst using Search Tempest and trawling through hundreds of ads over the better part of a year. I keep finding interesting stuff that way.