Here is a table of agencies where you can submit your content for commercial purpose. Mainly they are microstock sites where even beginner artists may try to make money (selling photographs, illustrations, etc.) - just take care when applying for membership - you'll usually have to send some of your best works.

You will also notice a few resources where accepting artists is marked as 'pro' - that means, you should have good knowledge and experience as, say, a photographer, and proper professional equipment as well.