We, the US Government, get all of our non-tactical vehicles from the GSA (General Services Administration, google them for all their bad press regarding waste & excesses). The GSA in turn leases these vehicles to the Army, Navy, Air Force, Marines, FBI, CIA, etc.
I looked up the GSA fleet site and did a search of the makes they offer for auction (they sell off older vehicles). The vast majority of vehicles are Chevys (1379), followed by Ford (569), Dodge (546) and so on down the list. I didn't see Hummer listed.