Medivac decals for any scale vietnam era huey

Fireball Modelworks makes a set that can be used on Vietnam birds too.  It is their FlatIron MedEvac decals.  Or you can make them yourself.

If it's the Red Cross you are looking for it would be very easy to paint on the bird or make your own using white decal material and painting the red cross on it.  If it's "nose art" or names you would have to probably make your own.  Check out the Armor section since the Red Cross is "universal".