I don't know exactly what fixed matches you talking about but it looks illegal. It is likely a bias or something like you know the outcome and going to bet a huge amount because you already know who's going to win the game. It most likely will be manipulated; it can be through the coaches, a team or a referees.
I don't think there's such a thing here on the forum; you can't able to find any casino here with biased matches and it's illegal.
It’s definitely illegal, so if the OP ever finds a deal, it’ll likely happen privately through DMs where they discuss the details --- like some kind of underground syndicate agreement for the service they’re after. As we can see, most of us are criticizing it because, while we might understand how it works, we don’t support these unethical and illegal betting activities. It’s just not something worth endorsing.