I don't have any leads for you but there's nothing rare about something you can buy new. The dealer and a few third parties including Peach Parts/Pelican Parts will gladly sell you one.

The #20 head was available for a time after the #18 and before the #22 head as a replacement for US turbo applications so don't count them out. If you don't mind, #17 head came on the 603.97s and is one to consider as well.

Sixto
87 300D