Preferrably from around the 1990s.

I assume you might like a Motorola such as the M2-RN-T12-1LL-120.  Unfortunately mine both failed though I was able to get the comparable Sylvania model as a replacement.  Although not vintage, I'll note that the GE GE240PSMVN-DIYB has nice slow startup, so that might be one to keep an eye out for.  Unfortunately I don't have much in the way of extras myself, though if you have any interest in magnetic rapid start ballast for 2xF40T12, I have lots of those.

I hear that it’s commonly the capacitors in those ballasts that fail. Since they aren’t potted in resin or tar, you can actually recap them if you so desire. I do want to obtain one of those ProLine ballasts as well, even though they aren’t that early.

The Motorola/Sylvania ballasts Patrick mentioned are cool .. both the 1-lamp & 2-lamp versions, as well as both T8 & T12 version.
(and various more modern GE electronic RS are nice too, plus quality seems good on them)

These Thomas - WWB-234-120-1 were one of my favorites, even though the quality wasn't good. I've never been able to find another though

These Smallwood - SEB 4H120 are another pretty nice one

I also heard of another brand that made pretty early electronic ballasts called EBT/Electronic Ballast Technology. Does anyone have any experience with them?