The information is on this link to Myles Barkman's site, but be aware that most Presidential related comms are digital/encrypted, and often very low-powered. If you are not within sight of the protectee, you won't hear very much.
http://home.houston.rr.com/freqzone/freqs/hou_frq2.html
Scroll down to the Federal section.
Your best bet for monitoring VIPs is by listening to HPD and the news media, although they tend to use telephones and are already prepositioned, so they don't use the airways very much.
Hope this helps.