just gave up 23 april toronto for 25 feb ottawa.Honestly just keep checking.
Earlier today, on like the 10th try (no kidding) i managed to get Toronto mid april..locked that down but ideally want something sooner (you can reschedule as many times as needed up until 3 days before appointment date).
When i got that slot there was also a Vancouver open for feb 7. also a Calgary for sept. so its just down to luck really.
I just kept refreshing at least once every hour..checking all the posts.
Re the question "are you traveling from another country to apply for a US visa in Canada?" Answer honestly (Even if its tempting to say no as that offers more interview dates). In December, my sister witnessed someone in Ottawa get turned away for answering no and not being able to provide PR card. That'll be considered misrepresentation and can go permanently on your file.
so its truly is just chance.
good luck y'all.