There's a lot of really solid advice in here. I'd vouch extra strongly for Instagram and Twitter. There's a ton of great artists in countries with a low cost of living. If you're a first-time dev and you can't afford $100 or even $50 per illustration, that's a good way to look.
Also always consider friends and friends of friends. You'd be surprised how far your social circle can go if you just ask around.
Great points. Initially we tried going with an artist from another country, but weren't satisfied with the results. It may be because our game is themed around American politics, and artists here can portray some of the more subtle humor we were going for. For games with more broadly accessible themes, like fantasy or sci-fi, going with an artist in another country could be a great way to get good art, support a talent, and save some dough.