Calcium shouldn't have a shelf life unless it gets wets and solidifies, but I would still get the smallest bottle. You will use it soooo slowly. Maybe for the multivitamin there is a "best if used by date", but still, smallest bottle you can get.
Get separate plain calcium (no D3) because you will need to use it several times a week. The multivitamin will get used on a much wider interval - once or twice a month depending on your chameleon species. I like the multivitamin with D3 so that I am only cycling 2 supplements.