Horze, they always have discounts on half their products, and there are also fantastic brands that have the same big discounts just depends on when you're looking.
Jeffers equine and state line tack
Statelinetack and horse.com
Chicks saddlery and horseloverz.com
Dover Saddlery has tack on sale or closeout a lot. They are normally cheaper
SmartPak has some good options. They have sales quite frequently too.