Jo-Ann Fabrics is a great place to buy your fleece and be sure to use a coupon!
Head on over to Jo-Ann Fabrics site, select Weekly Sales Flyer at the bottom of the site, enter your zip code, and search the most recent ad for coupons. Usually a 40 or 50% off one nonsale item. The coupons can be opened up on your phone so no need to print.
(Jo-Ann's also sells no-sew fleece blanket kits which is precut fabric in case you do not want to tackle measurements. These sell for around $23.)