Federal Frequencies?
Your best bet is to go to Google and just type in FBI frequencies or ATF,or DEA frequencies. Probably the best source out there at this time.
Also try Tootsie.com.
You will have to program the Frequencies and scan to see which ones are working in your location.
The Fes's tend to switch quite often, however some use the same Frequencies all the time. It's a search and identify mission to be sure and does take a little time.
Good Luck:
Iceman