Disclaimer: there is no "official" endorsement of any of these sites. Some I have bought from, most I have not. Before buying any watch, make your own investigation into the reliability of the enterprise.
Gisbert A. Joseph: Gisbert (in Spain) has some of the finest vintage watches on the web, or probably anywhere. You won't get them cheap, for good reason. Worth looking at the pictures at the very least. Many of them seem to be "new old stock" - never used watches which sat in jewelers' cabinets for half a century. Gisbert has the goods.
Girard's in Michigan has a good selection of vintage watches. Nice, affordable pieces. .
Northern TimeCanada's premier watch site. Good selection, check out their vintage Rolex..
Gold n Timeallows you to search by price. They're in Wash. DC.
Hamilton Electric. All the info you need about the first battery-powered watch, by Rene