Gunshows might get you a few loose rounds. You might end up with a few short and longs in the mix.
I have heard Old Western Scrounger out of the states has some of the oddball stuff from time to time. I think they make up small production runs when demand gets high enough. Never dealt with them, so I don't know what they are like to deal with.
Off the top of my head, I don't know of any Canadian sources to contact. Not much help, am I?
Had an old Stevens 25 rimfire years ago, but it ended up in the hands of an in-law that has dropped out of contact.
Just had a quick look at Old Western Scrounger. I didn't find any 25 rimfire. Sorry. You might try and contact them, and see what they say. They might have a lead to point you in the right direction