I go to the Casino approx 2-3 times a year with friends and every time I go I see the same 3 people at the poker tables. Not at the same table but they are always there playing poker no matter what day or time I go at. And the max buy in at these tables is 100 dollars and these three guys always have thousands worth of chips. So I assume they are pros who play poker for a living. Its literally their job to sit at the tables and grind grind grind. So to answer your question, I would say the casino.
In 2015 I played in a casino in Timisoara. And at the table I was playing, I was pleasantly surprised to see Torje sit beside me. He is a football player from Dinamo. The team in Romania.
You can find them everywhere. There are lots of serious poker players online and live. I think most of the time you can find them at big live tournaments.