I don't know off the top of my head, but if I wanted to find out, I would start by going to the Top 100 listings for Amazon's erotica categories and checking out the publishers listed for anything about billionaires. You can also just search "billionaire erotica" in the Kindle Store, and click through a bunch of titles to check their sales ranks and publishers. (and then once you have a bunch of publisher names, search for them in the AW Bewares board!! just in case)
I have a vague impression that a good portion of billionaire erotica is self-pubbed.