You can buy Meguiars and 3M products in Montreal. You just have to know where to look. It's definitely much more difficult that your US Counterparts.
Taxlady may be able to help you out as well.
I moved from Montreal a couple of years ago so the street names I am starting to forget a bit.
Try Lazer Auto for Meguiars. 3M I'm not sure of but I believe there is a AutoPartsPlus near Cote Vertu and North of the 40. It's tucked away on a side street. I can picture it but just forget what the names of the streets. However, if you would like, I could do a mapquest search and find the names if don't know of the places I am speaking.
Call around to a couple of the Paint Shops if they know where you can find some or better yet, try to find the main Auto Paint distributor in your area. Cmax/Napa is the one here in Toronto.
303 can be bought a Zellers.
Zaino - Easily found here in Toronto. I can send you some if you have trouble finding it there.
P21S will be hard to find. Gold Glass shampoo. Also a hard find for most of us. GC is good, but knock the E1 - I never used it but supposedly it is very good as well.
Porter Cable - Canadian Tire sells the 7424.
It's definitely a bit of a stretch for us for most products :-(