Hi Shadow,
Not big paying markets, but you might check out Wild Cat Books or The Pulp Factory, they are always interested in super hero novels. Plus, they know other markets for them. But, again, you're not going to see much money from these groups. I don't have their url handy, but google them. If you can't find them, go to my website and you'll find Wild Cat Books listed. Just click on one of their covers and it'll take you there. Uh, advise, though, don't mention my name.
"The Weed of Crime Bears Bitter Fruit!"