I buy men's shirts at Goodwill, but only when they are on clearance for a dollar. I need to start using them. I have SO many!
I have measured the square inches that I get out of a shirt and it is the equivalent of between 1 and 2 yards of fabric, depending on the size of the shirt and the cut of the shirt. Those XXL and larger shirts have a lot of usable fabric to them!
I always save the buttons and the labels.
A hint on to save the buttons and keep the sets together - at a dollar store I found a bag of 120 little zip lock clear plastic bags that are about 2 by 3 inches, just a great size to hold a shirt's worth of buttons. Then you don't have to sort through hundreds of buttons to find a matching set.