Buying Original Art and Music Covers

You can get nice original art at thrift stores, yard sales, etc. I have gotten into collecting thrift shop oil paintings most $5-15, and some of them would cost hundreds of dollars if bought elsewhere. I also have framed vintage music covers for their art work. (They are just the right size for standard diploma-size frames, too.) Where do you think antique pickers find their stuff to sell on to the dealers?