I don't know about apparel because I ride in my regular jeans and t-shirts
, but for tack I love Valley Vet Supply. Free shipping and high quality items! I guess I should also mention most of it is western.
I have a bit and cinch on order as we speak. I trail ride and I bought my helmet there, lots of minor tack items in the past such as cinches, bits, etc. And most of my veterinary type stuff too, such as dewormers, thrush treatments, detanglers, shampoos, brushes, etc. Oh, and Easyboots too.
I used to order from Horse.com/Country Supply, but ever since they got together with Stateline Tack the shipping has gone up and the selection has gone down. And what the do have is rather cheap quality, imo.
So Valley Vet all the way!