The Saddlery Shop are really good for hoof boots. They will advise on the best boots for the shape of your horse's hooves and hire boots out on trial. If you buy they take the hire cost off the purchase price.
What do you want to use them for? If you want "performance" boots, most people end up with the Easyboot Glove (which can be finicky to fit, though easy enough to handle if it suits your horse), or the Renegade (more forgiving of fit, dead easy to put on, but among the more expensive).
www.hoofbootique.co.uk are really helpful with advice, and you can hire a pair on trial from them.
The trick is to find a pair that will stay on well, and that is individual to the horse! If you want to go at speed/jump, the Renegades might be a good option.