First you have to identify which watches appeal to your wife. Is she more inclined to wear something a little more vintage or something newer. Then there's the size issue.
You have to find out how large a watch she will wear. Style is the all important factor for most women so you need to know if it's gold, diamonds, or stainless steel that rings here chimes. Once all that has been sorted out you have to identify what your budget will allow and go from there. New or previously loved. Womens watches take more of a beating than mens watches in the depreciation department. They drop like a rock as soon as you walk out the store door. There's not a lot of womens watches available on the different watch forums so if your're going the used route ebay is a likely spot to pick something up reasonable. Nobody fakes womens Breitling's so you're fairly safe in that regard. Sometimes you can find a smaller mans watch a woman will like. A Callisto, Sirius, Sextant, and a few others, are small enough at 36-37mm to make a nice watch for a lady. You can also post a WTB in the classified sections on the forums and see what comes up.