Ultra Magnus is now appearing on just about every major retailer. HasbroToyShop.com is now available and I would not expect it to last long. Unfortunately there is no coupon codes available other than the ones Hasbro sends out with purchases. These also do not include free-shiping unless you meet the $50.
Amazon also has had some limited supplies of this and with them you can get it free shipping and no tax. So without coupons Amazon may be the best way to go.