I recommend you go to the BHG, Bossk section, also the RPF Bossk posts, and the ones here as well.
I have posted how I made my Bossk on all three sites. I have only had my Bossk finished for about five months, and people have gone crazy over it. It won the "Best Star Wars Villain" at Celebration V last month, and the "Best Star Wars Character" at DragonCon last week. The Actor, Alan Harris, who played Bossk originally, was very impressed with the costume at Celebration V, and said it was the best he had ever seen.
There are five Bossk masks I am aware of, besides scratch built ones. The are occasionally offered for sale on the RPF and Ebay. A realistic Bossk is a very complex costume, and it took a lot of work to do it right. The suit is a 1960's vintage, RAF high altitude pressure suit, not just a "yellow cotton coverall". Check out all of those sites I mentioned. My article on how I builtthe whole thing is supposed to be published in the official Star Wars Bantha Tracks or Insider one of these days.