I go thrifting a lot, and while I'm searching for treasures I always keep an eye out for craft supplies. Some Goodwill stores will have whole bags of assorted supplies packaged together for a good price.
One of the most common finds is embroidery floss. I always grab any floss I can (as long as it smells good - obviously I avoid anything that smells like smoke or basement). Estate sales will often have large quantities of floss from someone's stash, in nearly mint condition, in just about any color you can imagine.
One cool thing I've noticed is that you can get an idea of the age of the floss by the price. The current price for a skein is about 40 cents, but I often encounter floss with price tags of 15, 10 or even 5 cents.
I believe floss costing 5 cents would have been made as far back as the 1960s. This beautiful thread has sat around unused for decades! I love the idea that it's been waiting for so long to fulfill its purpose, and now I'm using it to make decorations that will be displayed and enjoyed. It finally gets a chance to shine, after all those years!
FYI, here is what the purple skein was made into.
My shop: PeachPod