I would rather save aside money to buy quality fabric to make a quilt and make fewer quilts. Sorry, but if your choice is between buying fabric and buying food, I'm not sure you are in a position to be buying old clothing to make quilts.
Sure, I've made quilts from old clothing that is no longer wearable, but still has some usable parts. The quilt doesn't last that long in comparison to one using new materials. I chose to tie rather than quilt. You are still spending thread, batting, and binding, not to mention electricity to make that quilt. Bonnie Hunter, I love her, but she is using primarily barely used clothing to make her quilts - looks great, but ultimately costs as much as fresh yardage from Joann's (do the math).
If you absolutely are determined to use old clothes, then be a buddy to you local thrift store. Leave the stuff for those who need clothes to wear until the end of the season. At the end of the season, they are trying to sell off all the seasonal clothes to make room for more. If you show up on the last day of the sale, everything is often marked 75-95% off. They just want it gone. If your store manager knows you need a bunch of shirts, s/he might even give you a better deal. Last Spring, I let the manager know and was able to buy several boxes of sweaters for $8. My sister serged the sweaters into zippered cases to allow for padding. In turn, the finished cases were given to a rural animal shelter to keep the animals war and cosy while they await adoption. Obviously, you can make a similar deal for shirts.
Keep an eye out for estate sales. Quilters pass away suddenly and sometimes family members are too overwhelmed and simply let the stuff go for $5 a box.