I've bought a lot of my pants, shirts, vests etc from Wild West Mercantile in the US.
Even if you don't buy from them, it will give you some ideas, but as stewbud mentions, Apple Saddlery has/had a good selection of the kinds of things you'll want to try on to ensure fit: i.e. boots, hats.
Make sure you look at the Single Action Shooting website and download the Shooters Handbook for the covenants concerning the shooting categories and types of of firearms and clothing that may be specific for the category you want to shoot in.
I'm assuming you are in the Ottawa area.
If you are, I am one of the Trail Bosses for the Square Circle Wranglers and we are at the Eastern Ontario Shooting Club, 4570 Drouin Road, Cheney in the City of Clarence/Rockland.
We also have posses at Cornwall, Ruff's Regulators and a posse at Prescott,
I invite you to come to our matches.
Have a look at our websites for a look/see at our facilities and the dedicated CAS range at EOSC.