Do invigilators for test get paid or are they volunteers?

They get paid for their time, they're all TAs

Are you looking for one or do you want to be one?

Originally Posted by Tailsnake

They get paid for their time, they're all TAs

They're not all TAs... Some of them are just employed invigilators.

For midterms, 99% of the time they are TAs and get paid whatever they get paid for TAing normally (~$22/hr for undergrads and ~$40/hr for grad students).
For exams, 99% of the time they are external invigilators that are hired temporarily. If you have an instructor-supervised exam, you will have your instructor (obviously) and/or TAs.