I always still believe there is nothing called professional gamblers or bettors or whatever they are called online. This is because many of them that said they are professional either have other means to make money and they do not need to depend on gambling at all, but while gambling, some fall into trouble because they are not able to manage their money appropriately or because they thought they are professional gamblers.
There are plenty of them.
You can go and check reputable websites that provide tips, there are a lot of guys there with over a decade of history, that are profitable, if we go even to guys that offer just event tips, the guys dealing with racing post for example have had for the last years a profitable tip history, not by much, we're not talking here about doubling your bankroll but somewhere in the 25%-30% area for best years, depeding on your bankroll and actual capacbility to place all the bets so you don't lose out on random picks you would have been in profit. Of course, past experience doesn't mean a thing, and betting with them for only a few months might be a losing choice, but still, I will stand my pov, there are guys who do this as a living.
Alan Potts and Tom Segal, for example, are documented tipsters, both have two decades of tracked history.