I've seen this come up in the RPG.net forums before. They were generally kind to the person asking, but scoffed at the idea of someone charging for their DM services, or even making money at it.
https://www.wired.com/story/it...ndd-dungeon-masters/
That's a recent article on Wired magazine. It is possible to be a professional GM.
Critical Roll is another GM success story.
Professional GMs can recruit players from many sources. RPol.net, RPG.net, Google+, Facebook, Roll20.net, Twitch, Discord... anywhere you find people gathering to play RPGs, you will find a potential recruitment ground for players. You can advertise in the same places.
I am unaware of any training programs or certified college courses, but eh, that's possible in the future. It might exist now, but you'd have to go looking.
Supporting yourself? It depends on how you do it, I think. If you go the way of Critical Roll, and regularly stream on Twitch and YouTube, you can make a living off of it. But it's like anything else you do on Twitch or YouTube, you have to build your audience. It's not guaranteed, it's not easy, and it will likely take a bit of time to work up both a reliable pool of players and a consistent (and paying) audience.
If you go the way of hiring out your services, where the players are paying you (not an audience), then you are looking at an uphill battle, I think, it'll be much harder than going the digital route. If you are not willing to travel, then you have to have the space to have players come to you, and have all the requirements that entails (bathroom space, accessibility, food sources nearby, parking, etc.).
May I suggest that you go to Roll20.net, make yourself an account, and then look through the LFG (looking for group) forum? Look for the paid GMs, and send them PMs. Of the ones I know, I am pretty sure you'll find some who'll happily speak about their experiences. I can recommend a few, but I am at work and cannot get there from here currently. I can do so when I return home. You can reply here or send me a private message.