Bootfitter, always. you'll save a lot of pain and money. Shops also almost always will price match, summer sales should be in full swing, and tweaks/fitting changes to the boot are free.
If you are like me when I was growing up and 7+ hours away from a boot fitter, my advice would be to make it into a ski trip this winter. Schedule an appointment and go get fitted then spend the next couple days skiing/getting the kinks worked out with the fitter.
If that also is not possible, at the bare minimum you need to get your foot length, width, and instep measured. shoe size absolutely has no bearing on ski boot size. (I wear anywhere from an 8-10.5 depending on street shoe brand and am in a 25.5). With hard measurements and a lot of research, you can make an educated guess on what boots are going to be better for your foot.