The three main types of surplus 8mm you'll come across these days are Yugoslavian, Romanian, and Turkish.
Yugoslavian surplus 8mm is the most common these days. It is brass cased heavy ball ammo. Overall, it is good quality. The older production ammo (1950s) can be hit-or-miss in some rifles, due to hard primers. Some folks have to upgrade their firing pin springs, or attempt multiple primer hits, in order to get ignition. With my Mausers, I get about 1 in 20 rounds that need a second primer strike to fire. Older Yugo ammo is also known for the occasional case split, so be sure to wear eye protection. Newer production Yugo 8mm, up through the 1990s, is better. I haven't had any problems with my 80s or 90s-dated Yugo.
Steel cased Romanian 8mm is decent and available from time to time, but it's light ball, and most Mausers chambered in 8mm are designed to shoot heavy ball. It is surefire ammo, though. And there are fewer worries about cases splitting. It makes good plinking ammo.
Turkish 8mm is brass cased heavy ball. It can be okay, but generally fairly old. Expect occasional misfires and hangfires. There have also been reports of cases splitting, like with the older Yugo ammo. It pops up for sale from time to time, but in my opinion, it's the worst quality of the three common surplus 8mm ammos.