Many resorts offer testing on site (Tulum) or through pharmacies. To re-enter Canada your PCR test must be within 72 hours of the time of your crossing. So if you are walking across the border, make sure you have extra time for your commute, plus I would add 6 hours incase of any airline scheduling delay.
Public, free testing is a great option in much of the US, but takes a little research.
Miami-Dade County, Florida Nasal self-administered PCR 48 hour turn around. You can make an appointment for drive-thru or just show up for walk-in, depending on location. You must provide a local address for contact tracing, and simply use your hotel / accommodation address.
New York City: New York City Health offers rapid testing sites on the link provided. I went to the facility in Harlem, was very fast & friendly. The results were emailed same day and available on their health app, which they ask you to download and test before leaving. Again, I provided my local address for tracking & tracing.
Have you found any free sites for testing? Leave a comment and help us create a more extensive listing!