Gun shows usually have a plethora of 20 gauge ammo along with over inflated prices on .410 ammo.
.410 ammo not as abundant as the 20 gauge, but it can be had.
Since the 1980's 12 gauge had been a staple on the shelves at Steveston Hardware, Crappy Tire, Brighouse Hardware.
Those stores would have a few flats of 20 available in the late summer for the upcoming Waterfowl and pheasant season.
The War in the middle east played a part in the shortages as did the last three presidential elections imo.
https://ellwoodepps.com/hunting/ammunition.html#!/ammo_type=3322&order=news_from_date&no_cache=true&ammo_calibre_for_filter=3792-3785&p=1
OP, you being in Ontario I did a quick search with E. Epps and found a plethora of ammo and dam near snorted my morning tea when I seen the price @27.00 a box...