Tips on Frugal sunglasses?

My wife really needs sunglasses when driving. She usually just grabs a pair at whatever drug store or mega Mart she's at. She does tend to lose them, so I'm not looking for anything expensive.

I think she'd do well with polarized lenses, but they need to not change the color of the light (she rejected one pair that made everything look green).

What's the best way to find reasonable sunglasses?