Craftsy (when there is a sale) and MassDrop have the lowest prices that I have seen online. There is another website in Canada that has Aurifil on thread from time to time. It is called
www.tristan.bc.ca. There are other websites that carry it and run it on sale occasionally, Fat Quarter Shop and Missouri Star Quilt Company. I think many people like Red Rock Threads and Hawthorne Threads, but I have never found a deal there.