It takes a little work, but thrifted sweaters are a great place to source yarn. You can find unique and what might normally be expensive fibers, for a crazy low price. Bonus, you save a sweater from living the rest of its life in a landfill! There are plenty of posts out there showing how to take apart your sweater, how to chose the best and how to wash your yarn. Please defer to the experts, I'm an amateur with this! Here's a good post from How To Spin Yarn. Need more help? YouTube tutorials are great for visual learners!
When picking sweaters, think about this...