At the suggestion of a blog fan, I have started to add FBI frequency updates to the various agency pages at the top of the Fed Files blog. Since the FBI does not operate on a standard, nationwide channel plan, listing the various known FBI channels by US city should provide some idea of what frequencies are used where.
I have just started, so things may look a little sparse for a while, but I will be adding more as I have time. Also, if you have any FBI frequencies for your area, please pass the along and I will add them to the list.