Norm - Speaking from experience, I agree that buying a smaller size bike used IS difficult. There just aren't that many out there at any given time. I do agree that SHE needs to try out the bike for fit, but if you aren't adverse to buying off ebay or craigslist and then reselling it if it doesn't fit (my DH's favorite past-time!), then it's a great place to 'shop around'. Patience is a virture though.

The other thing to consider is how the geometries of all the different brands affect effective top tube length and how this compares to your wife's measurements. Once you know what works best for her in terms of the major dimensions of the bike (height, TT, seat angle, etc), it's easier to buy a used bike with a bit more confidence and improve your chances of getting a decent fit that can be dialed in. Of course, this doesn't take a lot of other comfort factors into consideration, but again...it's a start.

Good luck with it!