You will be joining the great but silent (up to now!) group known as "Green" quilters! And you'll be saving our beautiful planet in a very resourceful and useful way. According to the Council for Textile Recycling (
http://www.weardonaterecycle.org), the average US citizen throws away 70 pounds of clothing and textile waste PER YEAR.
I discovered thrifting when I underwent gastric sleeve surgery and went through clothing sizes really fast. All the clothing I wear now is gorgeous, some great top brands, and cheap! For my quilting, I have discovered that thrift stores have lots to offer. Here are some examples including some I think might surprise you.
Fabric - I cam pretty much tell when something is cotton. If I want to be certain, I do a burn test at home. If it beads up and smells like chemicals, it's a blend or total synthetic. But synthetic fleece makes a fine backing! I prefer all cottons but only because I can find plenty of those. That doesn't mean I haven't used a blend when I found one that worked!
Linens - Treasures abound. Obviously, sheets in all sizes, shower curtains, regular curtains, furniture covers, bed skirts, the list is endless. Head to this section first! Don't forget to expand your search to wool blankets. They make wonderful battings and come in all sizes. Just remember to label these properly for washing when finished. Wool trimmings can be "felted" for those who enjoy wool appliqué. A recent discovery is duvet covers! The label giving fabric content is usually found inside one of the corners on the button opening side. Most I have found are 100% cotton. This is a "twofer" as I can back two big quilts with them. Your hands will learn to find the treasures quickly. Most have labels for verification of fiber content if it matters to you. I like knowing!
Clothing - Men's XL and up cotton shirts! (OK, Large if you just love it!) Bonnie Hunter can "filet" one in 7 minutes! There's usually well over two yards of usable fabric in a big long sleeved man's shirt. Don't forget to take that fabric swatch with you that you have never found a compliment for. Men's solid shirts are a gold mine of colors! Don't waste time with women's clothing, there's much more fabric and choices over in men's. Also, wool clothing to felt for appliqué can be found throughout the clothing sections. Just sweep your hand along the racks as you walk along. You can feel the wool! Don't forget men's cotton and flannel pj bottoms. Most have never been worn. LOL!
Hardware - Clamps to hold rulers down. Also, small hand weights do the same thing. Shower curtain rods for rolling up fabric or flimsies or pinned tops ready for quilting - great for transport and adjustable. Tool boxes.
Kitchen - TRAYS! and BASKETS! Great for organizing what you've cut or pressed units! Glass jars with lids for bobbins and threads. Very decorative and useful.
Office - Desk chair. Yep, my sewing machine chair is being recycled and it's great! Command hooks! Rolling carts for your traveling sewing machine and stuff!
This is just the tip of the iceberg. The list is endless! Have fun!