I've been contacted by the IRS for supporting docs for a SAR that was filed. They would like all the docs but specifically asked if I could send them the copies of specific docs by fax, because they need the asap. Is it ok to fax it to them?

Check with your bank's legal council. Before I would give the supporting documentation I would ask for a subpoena. A couple of years ago I asked this same question and both the teacher and our bank's legal council said, Yes provide the info but only with a subpoena.

You don't need a subpoena to provide supporting documents to an appropriate law enforcement agency.

http://www.fincen.gov/statutes_regs/guidance/html/Supporting_Documentation_Guidance.html

As far as faxing, I don't normally fax supporting documentation but will send it overnight once I've confirmed the request is coming from an appropriate agency.