Actually, they make a new one every day.

A contractor has a van with many zip-up-the-back jersey's in every size. He also has screens of all the primary sponsor's logos. Before the podium ceremony every day, he makes up a winner's jersey with the appropriate logo.

I assume that the respective teams also have made up leader's jerseys and even complete kits with all of the appropriate sponsorship logos to be worn during the next day's stage.