I agree with the above suggestions. Borrowing saddles to see what fits is a great way to start, then you can take measurements and find out the specifications from the saddle's owner to look for something online with the same specifications. Look for a reputable seller with a known name. There are many cheaper saddles on ebay
and the such and they frankly aren't worth a penny.... I have bought a few myself and was very disappointed. Shiraz is a company that says they are made in USA, but every thing I have ever gotten from them have had a stamp or tag that said made from India and the item was always a piece o crap, so beware of Shiraz.
Always find out about the return policy as there is a chance it won't fit your horse. There are many online stores like Smart Pak and Dover that actually let you try the saddle out. They are a little more costly, but it may be worth it to be very sure your saddle fits.
If you are on a budget, Abetta makes some decent synthetics. I bought their trail saddle and my horse and I both love it. Super light, super comfy and super easy to take care of (you just hose it off). I like to recommend them for people who mainly go on trail and don't do a lot of showing, or for a "fun" saddle that you don't have to worry about messing up if you decide to "swim" your horse :)