You would think the control tower phone numbers would be listed prominently and conveniently on the FAA website.
You would however be wrong1.
Your best bet would be to contact the Flight Standards District Office that covers your area - they can get you in touch with whatever other branches of the FAA you might need to talk to. You can also get tower phone numbers from Flight Service (1-800-WX-BRIEF).
AC-U-KWIK also has tower phone numbers in a conveniently searchable system (punch in the airport identifier and it will be in the results you get), and the printed edition of the AOPA airport directory used to have tower numbers as an appendix (I'm pretty sure it still does).
1 – Actually that's not strictly correct: some regions publish tower numbers prominently. Most don't, or my search skills aren't good enough to find them.