The mints have lists of authorized dealers. For example https://www.mint.ca/store/mint/customer-service/bullion-dna-dealer-locator-10900014
It's also a nice place to find a lot of dealers to choose from.

APMEX is listed under US dealers
Kitco -> Canada, US and Asia
Canadian PMX -> Canada
etc...

Also, know that the mints do not sell directly to individuals, but to their authorized dealers, who in turn sell the bullion to individuals. Take your time to shop around, prices vary greatly. For instance, Canadian coins will be cheaper from Canadian dealers, while they're more expensive at American dealers, and vice versa.