Howdy hero! Welcome to TDH.
The very first question you need to answer is which version of Boba Fett do you want to put together? Pre-pro; Empire Strikes Back; Return of the Jedi?
Once you've answered that question, then you can researching all of the components and start assembling the costume. I think people have already given you good advice on where to look for a good helmet. I can give you two websites that I strongly suggest you take a look at. Both were very helpful to me ... granted I am working on a Return of the Jedi costume:
http://www.tk409.com/fett.html
http://www.tk7602.com/bobafett/
The other thing you should think about is how far do you want to go with this? Meaning how screen accurate do you want it? I will warn you that the answer to this question could easily burn a hole in your wallet! It is something I call "the Fett Disease" in that I have become obsessed with upgrading my costume so that it looks as screen-accurate as possible. I started working on my Boba Fett on April 28, 2006. As of today, I am currently on version 3.0 of it, and it will become version 4.0 in a few days, and then 5.0 in late October / early November. I keep upgrading the components of this costume over and over again. So what I am saying is you may decide to call you costume "complete" when you have all of the components, or you may become as obsessed or even more obsessed with upgrading as I have become!
Hope this information is helpful.:cheers