I have never heard of a saddle fitter going shopping with you. I would DEFINITELY expect to pay them their hourly rate for that, but it seems like a waste of money. Most tack shops will allow you to trailer the horse there to try on saddles, or take saddles out on trial (you pay a refundable deposit). You bring a few likely good fits home with you, ride in them, and have the fitter there while you're trying them out to assess overall fit on the horse and with you riding in it. They don't make any adjustments to the saddle until you've purchased it, then they adjust as needed to get it to be the best fit for your horse. It may require more than one visit from the fitter if you don't make a decision and buy the saddle the same day they're out. Expect to pay them every time they're at the barn.
You're going to need to educate yourself some to narrow down on a few saddles to take on trial. First, sit in them. Are they comfortable for you? Don't worry about horse fit yet, find a few brands/styles that you like.
Once you've established that, then you need to do some basic investigation about whether saddles comfortable for you are likely to fit your horse. I don't know anything about western saddles, but this is a comprehensive overview: http://www.circley.com/wp-content/up...le-fitting.pdf
Start with a wither tracing. Here's how:
Once you have that done, bring that to the tack shop with you to see how it matches up with the brands your store carries and that were comfortable for you.
Also, don't limit yourself to the tack shop in your town. Do some research on online companies (selling both new and consignment) that will allow you to take saddles on trial (and be prepared to pay shipping costs). I don't know anything about western saddles, but for English saddles I have had great success with Fine Used Saddles. They will also likely want to see your wither tracing to help with fit. It will probably be costlier to go this route because of shipping costs, but in my experience, it will give you access to a better selection. It's still helpful to narrow down on brands/styles you like by exploring at your local tack shop.
One other note- like people, horses' bodies aren't static. Just because the chiro found the horse in a different condition than the person who worked on her in June doesn't mean either were right or wrong. My mare sees the chiro every 4 months. I know where she is likely
to be sore, but the assessment is not the same every time, because the horse isn't the same.