Go to a Minute Clinic or Urgent Care, get a check-up and certificate. I don't know how it is in your region, but I've had plenty of same-day visits in the USA. You might be waiting awhile and you may catch Covid-19 before you are done waiting (only half joking) but it should be possible...
The requirement is very simple and I cannot imagine a doctor refusing to sign it, it simply needs to say you are not sick or having symptoms at the time of appointment
https://www.samoagovt.ws/2020/02/tra...t-requirement/
If I were you I'd plan on showing up to a walk-up clinic at the time the doors open the day before your flight.