Size 8 hats??

[Daubs]

Cabela's used to make XXXL fitted hunting hats...thankfully I picked up a few before they discontinued.

I wear a size 8 hat (buddy says, "you don't have a forehead...you have a SIX-head!")

Filson makes a nice hat, as does Stormy Kromer. Any other options out there?

 

[Dr. Vette]

Stormy Kromer was the first thing that came to mind when I saw the title of your thread.

Even Google doesn't bring up much other than these:
http://www.sport-smart.com/hunting-camo-caps/sportsmart-caps-xxl-camo-cap-for-the-larger-head.asp#

 

[Colorado Cowboy]

I wear lots of Stetson style western hats. There are lots of custom hat makers out there that can do it. The real caveat is a brimmed hat. If you don't want a brimmed hat, then it will be lots harder.