it's worth it to keep an eye on tackofthedaytoo , it posts a few new items every day at massively reduced prices and shipping is fairly cheap
IhorseloverZ.com has super low prices on a lot of good brands, not sure if they are good with shipping though, I havnt had the chance to order from them yet
SmartPak! They have sales all the time, and shipping is cheap/free over $75
I like the Greenhawk website!
Horse.com is a good cite.
I purchased blankets and horse boots there
chicks discount saddlery.. they have a lot stuff, it's mostly western tack but they have some english tack and it's all reasonably priced and made well!!