the rubies web site is http://www.rubies.com/ but they don't sell directly.
just google for "rubies jango helmet" and you'll get a ton of results. you can also search ebay for them.
since this hobby is mostly made up of garage kits and fan made sculpts, it's usually not as easy as just going to a store and buying all of your parts. it usually takes a bit of detective work.
check the cargo hold part of this forum, and make liberal use of the search box here and you should be fine though 