Hello and welcome to the forum
I know of one company here in the states that supposedly can supply parts in the white, however I have heard that the individual that owns this company is having some financial difficulty as of late, so tread lightly before sending any money.
http://www.josephbrazier.com/browse.php?view=PARTS&category=TYPE&type=10Your other option is to do what a lot of guys on here have done, pick one of the few American classic box locks and do a custom project using a field grade gun that is in great condition to start with, you don't want to spend a lot of cash to get it in great condition to just to start your project.
H&H