Wire with multicolor insulation is still available here and there. I think you sabotaged your search by using nonstandard terminology. I’ve never encountered the term “tracer wire” even in old electronics texts, and even Stray Electron was reading between the lines to know what you meant.
The other question is whether you’re after solid or stranded wire. Multicolor insulation was mostly used back when solid wire was king for internal wiring. Nowadays we do a lot less point-to-point wiring, instead tending to use connectors and PCBs, and when we do use wire, it’s more likely to be stranded.
At my old work, we had dozens and dozens of reels of solid wire, mostly 0.6 and 0.8mm diameter (22 and 20AWG), in all kinds of multicolor insulation. (In fact, when we needed it, it was single-color that we struggled to find!) That wire was mostly from the 70s, IIRC. My boss let me take home “a few” reels, so I now have a literal lifetime+ supply of solid wire. If you ever come to Switzerland, you can have some. Or if you are near Maryland, I’ll bring you some the next time I visit! :p