Katanga for one is listed by Scott.
Stampworld can be a good resource. I do not know how thorough it is.
Here is an example of Stampworld for South Kasai:
https://www.stampworld.com/en/stamps/South-Kasai/
Other than that, when I find stamps from lands like you mentioned that are not in the catalogs, I do an internet search. Often you'll find a dealer's listings for the stamps.
As part of my single specimen or stamp for every country I do collect them, but not in quantity.
They are very interesting and some are rather legit and are postally used even if not UPU recognized.
To answer the question I do not know of any listing for just these areas exclusively.
I wonder if anyone here collects stamps from breakaway regions like for instance Eastern Ukraine (Donetsk area) or Transnistria. These regions act as de facto independent countries even though they are hardly recognized. Often one of the first things such wannabe countries do, after designing a flag, is create stamps as a sign they are genuine.
In the past one would regularly see stamps from Katanga or South Kasai in worldwide packets and I wonder if the same will happen with these Eastern European "countries".
Does anyone know where to find a list of all non-recognized regions that issue their own stamps?
re: Breakaway regions and their stamps
Katanga for one is listed by Scott.
Stampworld can be a good resource. I do not know how thorough it is.
Here is an example of Stampworld for South Kasai:
https://www.stampworld.com/en/stamps/South-Kasai/
Other than that, when I find stamps from lands like you mentioned that are not in the catalogs, I do an internet search. Often you'll find a dealer's listings for the stamps.
re: Breakaway regions and their stamps
As part of my single specimen or stamp for every country I do collect them, but not in quantity.
They are very interesting and some are rather legit and are postally used even if not UPU recognized.
To answer the question I do not know of any listing for just these areas exclusively.