Online. Jomashop, authenticwatches.com, ect. I'd see what price an AD can give you before you pull the trigger. I wouldn't overlook the possibility of buying second hand either. There's isn't really a stigma about buying secondhand watches compared to many goods. Old watches are very often highly desirable by virtue of being old, and are 2nd/3rd/n'th hand. My Omega Speedmaster is a 3720.50 model, hesalite (plastic) crystal at the front, sapphire crystal at the back and they stopped making those 15 years ago, probably had a few owners before I got it.
It's value will only go up. Let someone else take the initial depreciation.