OK. if you ask an optical place for plastic non-prescription lenses... will they do it right then and there?

Question

or do you have to purchase the frames and then wait for the lenses??

Answer 1

providing they have the equipment (edger), materials (plano lenses and frame) and staff to do it, they can do it then and there.

Answer 2

then and there.

If you have your own frames they will have to edge them

Answer 3

Depending on how busy they are, you may have to wait an hour or so, or come back.  Most places aren't going to drop everything and do your lenses on the spot.