Yes, there are many free tests that Canadians can use in the US that would be acceptable by the CBSA to re-enter Canada. The issue is that these usually take about 24 hours to get the results. You can walk into virtually any CVS/Walgreens/Wegmans etc. In places like Florida and many other states there are government run centers too.

The Abbott ID Now machine is now being used in many Walgreens across the country which provides results also acceptable by the CBSA and gives you results within minutes. This is also free. Again you can go on the Walgreens site to schedule an appointment and find out which ones offer this.

Interestingly when I called the specific Walgreens that I knew had the ID NOW machine and asked them roughly how long the results take, she said "about 24 hours". I suspect they are trained to say this either to set expectations, or to discourage people coming in who need tests to fly. Either way, based on what I read here, I took a chance and got the results emailed to me in about 15 minutes after taking the test.