I got it from Snack and Tack last time, it was the cheapest I could find inc. delivery. There may be cheaper ones online but most were out of stock due to the shortage. I will shop around next time I need a bag.
Is it low sugar? Mine has a sugar allergy so is fed on D&H Equine sensitive, Fast Fibre and Hi-Fi Molasses free, but has dropped a bit of weight recently. He's not thin, but he could do with a bit more cover and finding a low sugar high calorie feed is harder than you'd expect!