Starwarshelmets.com has some excellent paint ups of Luke's helmet.
That site also has some really good pictures of two real "stunt" helmets, and may be one or two pics of one of Luke's production-used helmets, but I am not qualified to say if that particular helmet actually was on screen or not. The actual paint job differs slightly between them.
There are a couple of screencap threads in the prop forum that might have HD screencaps of the helmet, and there is also
The Star Wars Archives forum which has a great collection of Star Wars images production and promotional pictures in varying quality.
If you want to buy a helmet:
All licensed helmets for sale so far have been kid-sized.
eFX is working on a licensed 1:1 Luke X-wing helmet, but I don't know when it is supposed to be available.
For information about kits, search the forum! You could also check out JediMickey's run over at Rebel Legion's forum. I don't think it has got much attention here yet.
You guys are killing me. I was hoping not to cut my new helmet in half to vac it.
You don't have to. You would have to add the mohawk anyway, and your real helmet does not have it. There are also details on the APH-6B that are filled in on the X-wing helmet -- and the edge trim of the particular APH-6B helmet that Lucasfilm used is molded into the X-wing helmet shell's shape!
I would try building a rig to hold the helmet so that it can be cast in halves, build up the mohawk and other details roughly with removable clay and cast that. Then cast your positive molds in ultracal (or other durable type of plaster) and perfect the details by carving away plaster from those molds.