You could do a search (there are many resources online) for agents who handle erotic fiction. Then, assuming they take your book on, it is their job to earn thier 10% (or whatever is negotiated) by finding a publisher for you. They tend to know the genre and the industry better than most (otherwise they would not be successful) so they can help a great deal. Just be careful of the deal you sign (applies also to when you negotiate a publishing deal).

You may find it problematic if your novel is already published online. Many publishers may see that as a potential problem as it is already published. Make sure you have the rights to it carefully worked out - i.e. does the company which currently publishes it online also own the print publication rights? Do you actually have the right to take your own work and republish it elsewhere? You may not, depending on the contract signed. In which case, a potential publisher will want to see evidence that this is not the case otherwise there is a risk of legal action.

Also note that there is a big difference between self and vanity publishing. Self publishing is generally better as the vanity houses tend to take a lot of money but do not do anything for it (most of them just print the book and do not distribute it other than sending you a load of copies to sell yourself) whereas self publishing you generally pay only for printing (3-4 pounds a copy, less for large orders) and they sometimes also have links with online sales likes amazon.