Have a look here for 501'st CRL's:
501st Legion: Vader's Fist
Shadowdale creations
is probably your best bet:
Imperial Forces
SDS/original stormtrooper also makes them and are cheaper (although the earcaps aren't 100% correct). I wear an SDS lid with my IN Trooper, but Ainsworth isn't allowed to trade in/with the US, so it's not really an option for you.