I'm pretty sure the (2) biggest in Canada are:
Bill's Truck Shop in Oshawa (Courtice), Ontario
(
www.billstruckshop.com )
and
Scott's Super Trucks, Alberta.
...I've dealt with both and particularly Bill in Ontario for years and he is very good, very knowledgeable and has some very good parts and a good system for shipping, etc....well worth the call.
His tech advice on whatever you buy is very good.
He has a very big variety of parts, new, OEM, second tier, and used original so you can pick up something to suit your budget relative to your need.
Again, well worth checking out
...my two bits.
Coley