Thanks for all the ideas, but so far no one has posted how they arranged, or could schedule, a Covid vaccine appointment online in Colorado, which should not be that difficult. I would like to do my part, but I am not desperate.
I walk over to the grocery store a few times per week. Maybe I will try the walk-up and ask approach each time I am there to at least arrange an appointment, if that is even possible.
Failing that, I will just wait a few months until the various vaccines are more widely available for, hopefully, walk-up vaccination like the flu shot that do not require any kind of appointment.
Failing that, I will just "forget it."
How many others, less persistent than I and those on Facebook or using Nextdoor, will reach the same endpoint?