Try one of these. I've not used either but have heard good things about both..
Redmans Rifling and Reboring
Randall Redman
189 Nichols Rd.
Omak, WA 98841
509-826-5512
LaBounty Precision Reboring Inc.
Cliff Labounty
7968 Silver Lake Rd.
Maple Falls, WA 98266
360-599-2047