This forum is the last place you'd want to look for fixed games. They exist for sure, there's an ugly side to every sport, and match fixing is something that every country fights to keep under control. And as attractive as Bitcoin might appear to be to facilitate match fixing, it is anything but that. Fixers don't want to leave tracks. Using Bitcoin is begging to get caught.
Fixes games are only happening in smaller leagues, because they can afford it due lack of regulation. Any of the serious leagues like FIFA have very strict regulations and I doubt anyone would be getting away with buying any of these players which are making millions. When you are making millions already you don't need to take these risks, but in the smaller leagues players go from bill to bill and sometimes they are pretty much forced to do it to make some extra money. In any case cheating is always sad, stick to real betting.
More common in smaller national leagues, true. Less monitoring, players and officials risk less scrutiny, and can actually afford to get caught. I've seen it in my own leagues. But there's also a risk at the highest levels. The bigger the sport, the higher the level, the more money is involved.
But good advice to just stick to honest betting. Even if you somehow find a way to get into the rigged industry, you're entering a rabbithole that you won't escape from unscathed. What happens if you bet big and win bigger from a fixed match? You'll always be marked by organised crime, you'll always be eyed by the authorities. It never ends well.