The best way to find a reputable adult modeling agency is to check the major search engine's. You then narrow it down to agencies which are well known and have been in the business for a while and then contact them for requirement's and hopefully work.
I would go online to find this kind of agency. Google, yahoo, and any other search engine I could find. Most will be found in large cities. New york and Chicago are just a couple to check out.
There are many sources you can go to such as google. You may want to search for modelling agencies and targeting your search to specific agencies in your region.
A credible agency will not require money upfront for photography sessions or for agency fees. The best places to find modeling agencies are with retail establishments such as Macy's or JCPenneys because they hire for their own ads.
first of all the internet is the best place to find any thing, so modeling agencies is no exception. and if that doesn't help you should hit the big city. ny or la.