Try this link, for a pretty extensive (although probably not all inclusive) listing of certifying agencies, and their contact information.
http://www.kashrut.com/agencies/#__AU
Note added by moderator, on this website is printed (but might be missed):
“Inclusion of an agency on this list does not constitute an endorsement by Kashrut.com of their supervision or standards.”
this is just a listing, not all are necessarily accepted heksharim.