All flags are located in the Area 51 forts, however, when creating a mission there, the message “there is no T.O.E for this unit; spawning is unavailable.” is located where the unit list would be (first pic)
The center facilities, the most convenient for quick testing, are all tied to the Camps at the south of the island (Camp Victoria, Gretchen, Yvette). These camps do not even have Garrisons in them, making a mission is not possible at all (pic 2)
Anywhere else on the map (nowhere near as convenient as Area 51 in time spend testing/location) are Garrison-only, but those lists are limited and do not include all units, most notably, no tanks at all