just have to find a random BOS corpse with one on it I think, the BoS vendor might sell one, I didn't check however.
Well, you can raid the Airport and take the uniforms off anyone you kill.
They don't even seem to hold it against you long term (every time I go back they wait for me to open fire before shooting me).
an easy way to get a lot of them:
The hood you can get after completing a certain quest by killing a certain someone for the brotherhood of steel. Also, the vendor on the prydwin sells regular outfits.
If you mean just the regular orange BoS uniform, I believe Proctor Teagan sells them. I've found a couple on dead BoS soldiers though. As you progress through the game, encountering BoS soldiers as they're engaged in various battles becomes a lot more common. Some of them are bound to die. I've gotten a ton of T-60 power armor this way as well.