Check to see if you have a local vacuum/sewing machine repair in your area, they usually carry a walking foot for most machines.. you need to get one that fits your specific machine..
I find it interesting that the cottonc fabric is the one that is bunching , instead of the stretchy T-shirt fabric, try changing the adjustment on your machine that puts downward pressure on the presser foot...not the thread tension, but the presser foot tension.. look in your manual to find this adjustment, it is very common to change this for different weights of fabric.