I'm hoping someone who actually visits the Las Vegas boutiques will jump in (I've never been) to offer more specific advice. But, I'd call whichever is the largest location, and ask to speak with someone in the jewelry dept. I'd have on hand the specifics of your watch, as well as the length strap you need (for example, my Cape Cod double tour straps are short vs the standard regular length). The current straps offered are also quick release...which I imagine would work for an older style watch, but may also be something to ask. And, the straps do not come with a buckle. If this will be an alternate strap you'll use (vs a replacement) you may want to ask if they also have the appropriate buckle in stock.
Another thought would be to ask this question in the Las Vegas shopping thread, where you may get advice as to which location might be the most helpful. Good luck!
https://forum.purseblog.com/threads/las-vegas-hermes.673194/