Last time I flew into SNA, they were giving out free COVID test kits with shipping labels inside to send to their lab. It's a PCR lab test which should meet the specifications. However, some airlines are huge sticklers for making sure you have COVID test results from labs that specifically do them for international travel. These tests will typically run you between 100 and 200 USD.
If you happen to be at LAX within the 72 hour window, you could do the COVID test there for 125 USD. They have 1 hour PCR tests that cost 199 USD so you could even arrive at the airport a bit early before your flight to get it done.
The most convenient for returning to Canada would probably be the Switch Health RT-LAMP test that costs 149 CAD and is certified for returning to Canada (you have to join Aeroplan with Air Canada first which is free in order to get that price). You order the kit before traveling to the US and bring it with you. You do the test while videoing with their representative. Just be aware that you need a really strong internet connection.