Unfortunately, there isn't one that is both reliable and accurate.
There are only two makers of the X-wing helmet right now:
Richies Armor and
Dreadnaught Industries.
The guys behind them are also on the board. ("richiesarmor" and "Wakal").
RA's helmet is more accurate, but the quality is flaky. Some people have received great items, but others have not.
DI is less accurate, but is generally of better quality. However, they have been known to need more time before they can deliver.
There is a guy on Rebel Legion's msgboard: "Jedi Mickey", who is planning to do a run of his own sculpt. The accuracy has been somewhere in-between RI and DA, but he has yet to proove himself as a viable source.
Btw, I wish MR Laws would make them. The TIE-fighter helmet is like the X-wing helmet with additional pieces, and the X-wing pieces on MR Laws' TIE helmet are more accurate and of better quality than anything from any X-wing helmet maker that I have seen, except maybe SDS.
I also suggest that you check out Bingo Bongo's site:
http://www.starwarshelmets.com.
There you can also see images of the officially licensed helmets.