I think one is included and built into the helmet. At least it has been in the costumes I've seen in the past.
I don't own any full sized costumes myself, but someone I know that has made some fan Star Wars movies owns a few including Vader and Storm Trooper suits.
If I were to buy any I would want to get a Boba Fett and Storm Trooper outfit. But these all cost around £600 each, so not cheap.
Firebox is a cool site. I've been receiving their catalogues for some time now and they always have a lot of great content.