Glacier Bay White Medicine cabinet with mirror only came with one white shelf. How do I get the other white shelf?

Check the local Home Depot, it is one of their's.

Call the people you got it from and inform them that they screwed up.

Home Depot will take care of the problem, but it will take time. Before installing anything from the big box stores check to make sure you have all the pieces. If you don't bring it back and get another.

One of the ways is to buy only boxes that are factory sealed