There are several threads associated with Federal monitoring in the RR DB. You can review these if you like; however, this is a good place to start: MT Fed Files
The author of this blog has a column in Monitoring Times magazine, Fed Files. You can find Monitoring Times in many chain book stores such as Barnes & Noble. Monitoring Times focuses on a variety of radio-related topics. What interests me are Scanning Report and Fed Files.
From what I understand, Federal entities don't have to go through the normal FCC pathways for their operating frequencies. This is where the fun begins. There are various search limits for the federal bands, since they can show up in odd places. Again, there are many threads associated with these scan limits.
For instance, here in Waco, Texas, the Feds blend into the woodwork of existing radio systems here in town. Yes, there are some active federal frequencies in town, but most are encrypted & low power.
If you have have compatible RR account for downloading, you can find Federal frequencies in most counties. Many of these are listed under "Federal", but sometimes, you need to look under "Transportation" to find things like TSA and FAA, etc.
There used to be a book called the "Top Secret Registry of Government Radio Frequencies", but this is likely outdated. I also found this interesting "dated" book which has an interesting table of contents: Two-Way Radios and Scanners For Dummies: H. Ward Silver: 9780764595820: Amazon.com: Books
Truthfully, I believe RR and the aforementioned Monitoring Times are truly excellent sources of information regarding Federal Monitoring, my favorite area of scanning.
Since you didn't include what type of radio you are using or the programming software, I will share my preferences. I prefer using Win500 for my programming software. I also use a GRE PSR 500.
For most Federal Frequencies in "conventional mode", I will set my mode to "Auto" & set squelch to "search." From what I'm understanding, most of the DEA frequencies use NAC 156 according to the aforementioned author of Fed Files.
Also, remember to consult the "Nationwide" frequencies in the RR DB.
In my opinion, Homeland Insecurity has not been to kind to Federal Monitoring.
Good luck. "Onward through the fog."