I agree with the not Petsmart mice. I know Petsmart Canada probley uses different suppliers. I purchased a little Normal cornsnake in January, she was on sale and I was given a the staff discount because they gave away a hamster that was being saved for me(It was from an accidental litter in store).
Anyways the little corn went down hill quickly, her first stool was full of worms. Vet bills, dewormings, refusing to eat, assist feedings, scenting rodents and lots of grey hairs. Naos the snake is finally eating on her own and gaining weight 5 months later. I am not 100% sure if she got the worms from the store's stock of mice or if she came infested from the breeder/supplier.
It takes 3 months of freezing to kill worms in rodents.

I would try posting an add online looking for mice, I breed my own mice and I supply a couple local snake owners as well. I don't post mice for sale, but I do respond to want adds. I have even set a couple people up with breeding colonys. It's very hard to find female mice here (well Petland has them for $14 each).
If you have any friends with snakes you could always split an order and have some shipped to you.