There's also a supply issue, right now Pfizer is in short supply (in Ontario at least if not Canada as a whole), so people getting their second dose are, apparently, more likely to receive Moderna even if they had Pfizer as the first shot.
Media in the U.S. are reporting that mixing vaccines has been show to be safe and effective.
Where I am, I had to set up an appointment at some website when I became eligible. There was a choice among many clinics, but they didn't specify which had Pfizer and which had Moderna.
However, one was supposed to set up both appointments at the same time, and for some clinics, the timing was three weeks apart (must be Pfizer) and for others, it was four weeks apart (so Moderna).
I signed up for Moderna, as it was available first.