You may have to wing it. That is, figure out your felted gauge and then follow an unfelted pattern, enlarging it (per your measurements) as you go.
I've started doing this for a pair of felted moccasins for my husband. The key seems to be getting good measurements of your gauge swatch before AND after you felt it (also keep track of how many times you wash it and for how long each time, so you can duplicate this on the final product).
A hat isn't that big of a project, so perhaps if it doesn't come out right the first time, you could give it to someone with a smaller head.