Up here, we can make a "mold" of the horse's withers/shoulder/back and take it with us to the store. A knowledgeable sales person should be able to help you. Also know the breed of the horse, if not a grade horse and take conformation shots with you.
Before you buy, ask if you can return the saddle if it doesn't fit properly and what the terms of that return are. Sometimes, it's a simple matter of making sure the horse is very clean, put a thin fabric under the saddle and tack up the horse. Check for fit and you may just be good to fly right off the bat.