I am a member of the ASMP and they have a find an assistant search. I believe it is open to the general public, however I am not sure.
Beyond assistants, the ASMP is very much like a big family and I have never had an issue with picking up the phone and calling a fellow member for advice. This is not say they always have time to talk, but every fellow member has always been affable and helpful.
So typically, with an out of area shoot, I reach out to members in that area to see whom would be good to contact and hire.
We also have a closed forum, open only to members, that is very business oriented. Things like this are commonly posted there too, along with several other topics and discussions you would never want a client to read. Hence the advantage of it being closed to members only.
I would recommend joining; there are many other benefits as well.