Thrift store sweaters can be fun (with a few caveats).
Most I've seen, especially of good fibers like cashmere and merino, are knit from fine yarn - often multiple strands and not plied. It's okay, but a pain to keep those strands together when washing and knitting. Or, like the Land's End cashmere sweater I took apart, it's one fine strand. I have a LOT of nice lace weight cashmere, but it's not something I'd want to use to knit a scarf...
Watch out for cardigans as they may be steeked - often they'll knit the sweater in the round, then slice it up the middle and add button bands. Take that one apart and you get lots and lots of short bits of yarn. With enough practice, you can usually tell the difference.
I live in the south, so most sweaters I see are either acrylic or cotton. Multi strand cotton (8 strands!) is NOT something I ever want to knit again.
There are forums on Ravelry and several blogs dedicated to the art of deconstructing a thrift store sweater. I've also seen Etsy shops full of yarn harvested from sweaters, if you don't want to search, take apart, skein, and wash yarn for yourself.
If I weren't so ticked off at Knitpicks for their big credit card breach they ignored, I'd recommend them. As it is, I would only use them with paypal.
Also on Ravelry, people will often sell yarn they've decided not to use. You can check reviews of yarns there to see what people think.
Malabrigo Rios is my current favorite for worsted weight yarn. Beautiful colors and it's washable. It's not big box store cheap, but it's not too bad for a very nice yarn. Solid colors - Cascade 220 superwash is good.
Life's too short to knit with bad yarn...