Yup, online pay sites for por.. oops, erotic art , are making enough money that they dont argue much about pay. Its not great money but some dont demand great quality either.

I have done a few book covers which I got paid for yay but they were not used for the actual book, just some concept art. 

There was an archived chat  at Artzone with some sources for commercial work.  Mainly I think landscapes and posters but it all pays something.