Hi Frankie,
I got my blue EMS pants (2 pair) that have slotted buttons at a second hand clothing store (Value Village), they were inexpensive at $7.00 a pair and are my favourite pants. You can also fine Canadian Forces clothing at these stores for discount prices.
I just check the label in my EMS pants and they were made by Surplus St-Gilles in St. Gilles Quebec, is that anywhere near you in Montreal?
I Googled St-Gilles Surplus and came up with:
St-Gilles Surplus Inc, 2749 route 269, Saint-Gilles, QC G0S 2P0, Canada. 416-888-3220 Telephone, 877-694-3220 Toll Free, 418-888-3145 Fax and info@army-surplus.ca
I could not find a website but most of the information was in French so I understood little of it.
This surplus store may be able to provide you with the slotted buttons you are searching for.
Good Luck,
Mike