After Googling around trying to get replacement AprilAir filters I came across www.ontariofurnacefilters.com whose price ($45) was much better than I could find elsewhere ($60). A US supplier had a lower price, but shipping brought the cost to well over $60.
ontariofurnacefilters.com ships for free via Canada Post. I ordered on a Tuesday afternoon and got my filters (along with a pen and a thank you note) Friday morning.
I would advise anyone who needs filters to try these folks.