I did a couple of images for a guy who was writing a book about space travel. He’d seen my stuff online, and mailed me to ask if I’d be willing to create some illustrations for his book.
My client was happy with the images I turned in, paid promptly, and generously paid more than the sum originally agreed, so it was a very positive experience overall. That said, if looked at in business terms - i.e. time spent vs. amount earned - I probably just about made minimum wage. 
Luckily, I was doing it mostly for the fun of it, so the money came as a nice bonus.
If you want to make money from your renders, a useful first step may be to set up an online portfolio of your very best work, and work at making sure that it can be found by people who might be interested in hiring you, or buying prints, or whatever. After that, the key to success is either (a) being so good that you can command very high rates per piece, and/or (b) having your workflow and technique optimized so that you can turn out new work to order very quickly and efficiently.
I’m not sure where the best place to pitch your skills would be. One thing that occurs to me is that there are an awful lot of people self-publishing ebooks these days. If you could do book covers that would help their work stand out, you might be in a position to make a lot of small sales.
