I usually Google JoAnn Fabrics coupon to find websites with coupon codes. (Also do this for other online companies before I place an order.)
Also, you can go to
http://www.joannfabrics.com and enter coupon in the search box. It will come up with a FAQ page. At the left, scroll down to "weekly sales flyer" and click on that. It will show you all the current flyers and will also give you an opportunity to sign up for their email notifications.
There's probably a more direct way to sign up than this, but the above works.