Whats something that can keep your shirt form outlining your while not wearing a bra?

Im being straightforward here and my mom and I are planning on going bra shopping soon but haven't found the time. Im in desperate need for something that can keep my shirt from outlining my cause I bet everyone can notice that Im not wearing a bra and its kind of especially embarring for gym cause im only wearing a cami then. Anything you can suggest that I can wear temporarily?