I opened this topic to answer some questions on saddles for gaited horses; since this comes up so often, it deserves a topic of it's own!!
Your BEST bet for saddles is to buy a saddle made specifically for a gaited horse. Saddles made specifically for gaited horses are built with a wider tree, and the front of the saddle is high and wide and drapes over the horses side rather than gouging in the shoulder area at the points of the tree. They have a wide gullet, and the Western gaited saddles have shorter skirts to accommodate the swing of the horses back legs. Walking horses have sweeping movement behind (huge long stride), and a lot of action in the shoulder even when barefoot. Think of when other breeds do an extended trot, where they really reach up front; walking horses do that naturally. And my walking horses mostly have wide backs, and require a wide or double wide tree.
The best saddle for the money is The Tennessean. They start around $800 for a work saddle and get more expensive as you add silver in the show models. The Tennessean comes in both Western and endurance styles. I have two show Tennesseans, and we dont show in anything else when showing Western. The saddle is so balanced that even when my girth has come loose, the saddle stayed in position and I didnt fall off. The difference in the movement and gait when riding with the Tennessean compared to riding in a saddle with quarter horse bars is phenomenal. It frees up the horses shoulders, and maximizes the horses performance. The Tennessean is made by Crates, and they are sold by National Bridle Shop.
With some walking horses with a not-too-wide back, you can ride in a saddle with quarters horse bars, like a Big Horn. Quarter horse bars and semi-quarter horse bars are how they refer to the tree size in ordinary Western saddles. I think there is also a fit called Arabian bars, sized to an Arabian. Most western saddles are built on quarter horse or semi-quarter horse bars. We have a couple of Big Horn synthetic saddles that we sometimes train in, but they really dont free up the shoulders enough to allow the horses to maximize their gait and encourage them to reach from the shoulder. I rode in a Big Horn saddle for many years until the Tennessean came on the market.
There is another line of saddles built for gaited horses called Tucker. They have some really nice saddles, but in my opinion, they are built more for a man than a woman (I got terrible saddle sores when I rode in one), and you cant sit in a balanced position like you can in a Tennessean. Different people have different preferences, though, so Im mentioning those too.
Almost any English saddleseat saddle (flat saddle) will fit a walking horse, but if you want to seriously ride, you need to invest in a good one, like a Cliff Barnsby. They are very pricey, but the balance is superb. In a saddleseat saddle, there is almost nothing to it, so the balance is incredibly important if your horse starts to rear and buck. Sometimes you can find a good used one, and many saddle shops will take a trade-in on your old saddle.
In a dressage or jumping saddle, the width of the tree is incredibly important. The best multi-purpose English saddle Ive found is the Stubben gaited saddle. If you need to start out with something less expensive (my first jumping saddle cost $100 and I was awfully happy to have a saddle of my own!!!!!), just make sure the tree fits the horse.
Whenever you try a saddle, make SURE that the saddle company will take a return. Put a clean white pad or a white sheet under the saddle, ride in the saddle, and then look at the bottom of the saddle pad or sheet. If the hair on the pad is evenly distributed along the tree, then the saddle fits well. If you have big patches of hair only by the shoulder areas or by the back of the saddle, then you know the saddle is pinching or out of balance. If this happens, send it back and try another saddle.
The other test is how happy and freely your horse gaits in the saddle. If he doesn't slow or stop, but opens up in his gait and goes very smoothly without breaking, then the saddle is probably right for him. The other thing is your balance, as your horse canters and does manuevers, if you feel pushed forward, pushed backward, or about to fall off, often the saddle isn't balanced well. If you feel secure, it is probably the right saddle for you!
Here are a couple of examples:
Gunslinger is wearing the Tennessean; the saddle doesn't constrain his movement.
This is Flashcube in a Big Horn; see how I'm kind of rolling my weight back because the saddle doesn't fit as well.
This is Moe in the Stubben gaited saddle. My saddle pad is too far forward, but you can see how easily I'm sitting in balance and how relaxed he is.
This is Sling in the Barnsy saddleseat saddle. Again, note the balance. My hands are too high, but I'm really good balance--and that was really important as Gunslinger bucked and jumped in that class and I didn't fall off!!