Not sure what you consider best, but I assume price is a big factor. *Just noticed you wrote online but I'll just keep everything I have here.

You might find a used phone section in your local thrift shop.

Someone is probably selling new and used flip phones on kijiji, ebay, fb marketplace etc.

For buying new, Best Buy is available nationally. I've seen refurbished phone shops but most carry only smartphones. Wireless carriers can let you buy phones outright with no plan but they are overpriced.

Your best options are going to be from Japan because they have the biggest dumbphone market in the world, so companies still design new ones. Your best option will be through smaller 3rd party sellers or directly from the manufacturer's site. Many Youtube videos in the dumbphone community can give you better examples. Just be careful about getting a phone that is 4G or 5G because 3G service and under is getting cut everywhere. Also, asian networks use different frequencies than us in the Americas

I'm not Canadian, but visit often.

I'm not sure what the criteria is for "best way" for your situation, but if nothing else you can get these online from Walmart.ca.

Kyocera sells some great ones on their site but i needed a courrier service to transfer it from a US postal box to a Canadian mailing address

Disclaimer: US citizen here, but I did a quick search for ya and found this site...

https://www.bestproductscanada.com/latest-dumb-phones

Yeah I get it, yeah fuck Amazon and fuck USA, just figured I'd help..

Go to a cellphone store

Best Buy would be my first non-Bezos look. Maybe Staples.