I'd seriously consider trying to rebuild any that he might have laying about. It's going to be tougher than going to the NAPA and asking for a rebuild kit but there are a limited number of seal dimensions in the world, and they are probably the same size as something of similar vintage but more readily available. Strip 'em, get them boiled out at a real machine ship, hone the bores, and get creative about finding parts...
His name escapes me now (senior moment) but there is an Opel racer in Columbia, SC who has many various Opels at his shop. I believe he's a CCR member.
I have a Opel Gt.
I get most of my parts from Gil at GT source.
Opelgt.com has a great write up on rebuilding and replacing the calipers for racing.
Here is a pic of my baby! http://www.opelgtsource.com/