Amos Advantage (Scott Publishing) prints Minkus Global supplements, as you probably know, but they only have the more recent ones. To add older supplements you probably won't find an actual dealer with them in stock. Although with today's "print on demand" system, you'd think they could just print them -- "on demand," wouldn't you? You might call them and see why they can't do that. With some demand, maybe they'll rethink their approach and start doing that?
The only other possibility is to search
ebay regularly and look for either leftover supplements, as you've done unsuccessfully so far, or buy a Minkus album that has those supplements -- and remove them. That's what I did to build up my very large multi-volume (40) Scott International album, combining older, but new-looking, supplements with brand new supplements.
I collect "only" to 1975 which I've found is more than enough for one lifetime. In fact, it may be too far, and now I think 1970 would have been a better cut-off date, or even 1960 before the largest part of the gigantic flood of stamps "mainly for collectors" was released many of which never saw actual postal duty. Of course, even with my 1975 cut-off date, I'm still free to ignore the 60s and early 70s if I want to -- except for staring at those blank spaces, of course!
One last thought is to print Steiner page layouts on blank Minkus album pages and use those. I've done that with a few countries in my Scott International and they look really good. I print the pages I need from the Steiner website or disk on standard computer paper, then take those pages to Fedex and copy them onto my blank pages (with the border and hole-punched, of course) one by one. Time-consuming, yes, but I get the pages I need and they look really good.
And one last thought that just occurred to me -- I've salvaged various water-damaged album pages (in my case, large Davo pages) by buying matching blank pages and photocopying the damaged pages directly onto them. The result is identical but again it will take time doing the photocopying. Or you can have matching blank paper cut in the right size by a print shop and maybe they can do the hole punching, too. Everyone needs a project, right?
Good luck finding those Minkus supps. or making your own pages.