Depends how cheap you want it? I check often and the lowest i have seen a 05-06 mustnag gt is about $14000 and the ones that cheap have 75000+ miles on them. If you look you can get a decent one for $15000-$17000 that has not been beaten. Also....I have noticed that when a "rich" person decides they want to sell their mustang gt, they have more money than they need and will offer their car cheap cuz they dont care if they lost a couple thousand. Good luck.
depending on the state you live in it will vary.classified adds used car.com,or local papers. But if its cheap its for a reason,those cars unibody rails bend easy in a collision.they can be repaired easily,or butchered out depending on where it was done.Collision re-builders love them for that & check the strut towers 4 cracks,due to torq reasons.if you don't know what your looking for have someone check it out b-4 you buy it.
I don't think there are any cheap 9005 to 2009 Mustangs. You might find one cheap that has very high mileage or that has been beaten into the ground. But other than that they have high resale value and are quite expensive.