AFAIK there are no M1903 or 03-A3 receivers being stocked by any dealer in Canada. I've been trolling the EE for a good Springfield receiver, action, or a rifle with a toasted barrel for quite a while and haven't found one for a couple of years.
Finding a serviceable military M1903 barrel is a big challenge. When you can locate one, expect to pay $400 for a USGI barrel complete with rear sight sleeve and front sight band. Criterion makes a replacement barrel with a military contour, but they do not come with a rear sight base. Locating and installing one of these is also a challenge. I don't know if they come with a front sight band or if they have a keyway milled in the barrel to accept a front sight key.
Assembling a M1903 or 03-A3 is the same as any other Mauser type. You need a proper action wrench and barrel vise as well as a finishing reamer if installing a new barrel. It's a good idea to have a selection of triggers, sears, and cocking pieces on hand to be able to set up a rifle with a proper trigger pull. The barrel has a precut slot for the extractor, so proper indexing is important. I normally look for an initial hand tight position of 15-18 deg off vertical before installing the barrel. The last one I assembled had too light a draw which required the use of a breeching washer in order to get a proper fit.
You can get all parts from Numrich, except for a GI barrel. Criterion barrels are a licensed item for export. Marstar has new stocks and misc parts. If you are trying to assemble a "correct rifle" finding a stock to match the original is very tough. Most M1903s were rebarrelled during the course of military service, so any GI barrel is appropriate. If using a Criterion barrel you are basically looking at a shooter/parts gun, so any serviceable part works.