How to pound chicken breasts without a meat tenderizer?
This might seem silly but, . . . I use my fist. I always cover my chicken with plastic wrap to contain the mess before I pound it. You can also glove up instead/as well.

I have found that punching it is just the right amount of force for the job. And the uneven surface of my knuckles acts as a meat tenderizer and breaks up the tissue just a little. (But not too much.)