I have plenty of US ones bookmarked, but struggle to find good ones through google for the UK. Any favourites?
http://robinsonsequestrian.com/ (Used to be better than it now it but has some OK stuff)
I wouldn't bother with Derby House personally.
Depends what you are after really, theres lots of smaller sites, or specialists, like this one is good for hunting stuff http://www.calcuttandsons.co.uk/cate...ing-attire/12/
or dressage http://www.dressagedeluxe.co.uk/
ClassicDressage. I had a very good transaction recently, owner is great with communication - no funny business.