Most of them are purchased at the thrift stores. When I shop thrift, I look for the top brands like J. Crew, Worthington, Izod and Eddie Bauer. Often at the thrift stores all sweaters are the same price. Why pay the same for a Walmart special when you can get a good name brand at the same price.
You need the basic colors. Navy, white and black lightweight cardigans are a must. For cooler weather I look for cable knits and luxurious cashmere. Add some argyle to the collection too. They look great paired with jeans and cords for a neat casual look. I recently saw the Kate Spade Afton cardigan. Love those white buttons! So I pulled out a plain navy J Crew cardigan I recently purchased at Goodwill and bought some little white bow buttons on Ebay for $3 with shipping and I'm going to swap out the buttons tonight! No way could I afford the Kate Spade version but I can copy it!