Yvert and Tellier or Stanley Gibbons are the best catalogues listing DPRK stamps. They define original and reprint issues.
The best way to collect DPRK material is postally used on cover or postal stationary. You'll never find reprints on postally used material. Post-1945 thru current material ON COVER or Postal Stationary is extremely scarce and brings extremely high-$,$$$ in Europe, Japan and elsewhere. The material offered at auction on "DPRK Occupied South Korea" (early 1950s) is RARE and priced accordingly, if and when available. The material is found addressed to Russia and Eastern Bloc countries including Czechoslovakia, Yugoslavia, Poland, Romania, Hungary, China (PRC), Turkey, etc.
Collecting DPRK is challenging but can be done, even by U.S. collectors. Material can be seen regularly in sales by Kellerher (Mike Rogers div), George Alevizos as well as other European Auction Houses. Technically, POSTALLY USED DPRK covers and postal stationary entires are not banned by U.S. Customs under the embargo. I know, I collected this material between 1965-80 and sold my collection through a well-known US auction house in 1983 labeled as "North Korea".
Information is available on DPRK material from the American Philatelic Research Library
The Korea Stamp Society (US) is an excellent source of information. Anyone collecting Korea (Empire, Japanese Occup, ROK or DPRK) should join. It is an excellent group.
https://koreastampsociety.org