I had looked at two 912E's and in the process bought a book covering 1974-89 911's, 912E's and 930's. I knew there was section covering wheels and 15x8 was listed. They were specifically listed as "optional rear" for 1974-75 Euro Carrera, "standard rear" for 1976 and 1978 Turbo, "optional rear" for 1984-86 Carrera, and "standard rear" for 1987-88 Carrera.
Unfortunately, no mention was made on offsets.
I looked through some Porsche ads and came up empty. I did find a few options on eBay.
Odd style, double-check offset
Fuchs, double-check offset
set of four, two are 15x8
I was also thinking why not consider a special order set of rims to your exacting specs.
These guys caught my eye, and you always struck me as being very exact with the details of all of your cars.
I have no affiliation or connection with any of these specific sellers. I did find searching for what you wanted challenging. Good luck.