Military Vehicles
 

I know to use guns in a movie you just rent them, but wondering, where do they get tanks, armored personell carriers, gunships?

One of two places...

(1.) They get the Department of Defense to participate, and the DoD loans them the vehicles/aircraft and accompanying operators/crew. (This happens very, very rarely, because the DoD will never support productions that have negative attitudes towards the military - leaving out most of Hollyweird.)
(2.) They rent the vehicles from special companies that keep a large inventory of military vehicles, or civilian vehicles mocked up as military vehicles. (This is how it's done probably 99% of the time.)

Army Trucks, Inc. is probably one of the biggest rental companies for military vehicles in the U.S.:

http://armytrucks.com/

Most of the military vehicles you see in American films come from this company, including the mock-up M1 Abrams tanks that you've seen in movies like "Hulk", "Cloverfield", "Jarhead", etc.

From what I know of, Stargate SG-1 apparently has a division of the Airforce created specifically for the show, when one of the TV movies required a sub, the navy borrowed them one, and apparently the airforce responded by letting them show off F-16s.

Richard Dean Anderson actually got air force honors once for positive portrayal of air force personell, wouldn't surprise me that they let them show off some jets.

Yeah, some branches of the military do send entire units to aid productions. "JAG", for instance, used to get enormous amounts of support from the Marines and the Navy, and I think there was a permanent liaison between the production and the DoD set up just for them (I remember one episode of the show where the production was actually loaned several MV-22 Ospreys, and this was before the Osprey had even gotten into service). Also, non-military related, the NYPD has a unit that is intended to aid film productions.

But generally speaking, the DoD is usually pretty cautious about who gets loaned its equipment and personnel. A movie like "Stop-Loss", for instance, would never received DoD support (that movie used Humvees and a fake M1-A1 rented from Army Trucks, Inc.)

Weren't there a lot of military support for the Transformer movie? Those Ospreys, F-22s, and that Spectre gunship was really firing on the set.