The link for Allan May's site didn't post right but you can simply type Allan May Mafia into Google and it will pop up.

Also fbi.gov has a vault of FBI files that have been released over the years, they are not all about the Mafia but there is a search feature and A-Z index.

Also www.maryferrell.org is imo the best site there is for researching the Mafia. It is based around the JFK assassination investigation but is full of FBI files on the Mafia.

I hope this helps.