I shop for 100% cotton shirts. I check the cuffs and collars for wear. I give them a good whiff to make sure there is not an odor I don't think can be removed by the washing process. I stay away from flannel shirts, but I like the wovens. The shirt plaids are reversible, so if a shirt is faded on one side, you can turn it over and use the other side. Larger shirts give more fabric, but if I find a plaid I like in a smaller size, I will buy that. I look for the tag sales. Some of the stores have a color tag sale of the week. If the item has a certain color, you get 50% off. I also have collected 100% wool skirts for felting.
I have made rules for myself. If I buy it, I must take care of it right away. For shirts, I wash them and cut them apart, saving the buttons. ( I'm not sure what I will do with these.) For the wool skirts, I take them apart and felt them in my washing machine and dryer. Preparing recycled clothes is not really an easy task, but you get a lot of variety with the shirts especially. Most of the cottons are a bargain compared to quilt shop fabrics. The wool is truly a bargain. My local Salvation Army sells most skirts for $.99. I bought several the week blue tags where 75% off. $.24 for about a half yard of felted wool after the washing process. Some skirts, more.
This is work, but I enjoy the hunt and the fabrics you can find.