I know a guy who runs a small press, publishing games and books.  He goes out and trolls the graphic forums and sites looking for people who do interesting work that matches what he's looking for that week.  Sometimes it's space, sometimes it's erotic, sometimes it's military, etc.  Then he emails them and tells them what he wants and asks for a price and timeframe.  He's looking for prompt, professional responses that show the person is willing to do the work at a reasonable price in the time frame he needs.  Color cover work pays better then BW interior art, but you're not going to get rich on occasional $100 to $1000+ jobs.  But commercial sales means that your computer, your software, your 3d models, travel to siggraph etc is all a "Business Expense" and hence deductable.