I've been using "Cudas" for riding. These are polycarbonate-lens saftey glasses that are available for under US$10, and I get them at an industrial-safety place locally. I did a quick Internet search and found a place that shows what they look like, along with many others:
They are available locally in clear, amber, dark smoked (note: quite dark) and mirrored. Even the clear tint blocks ultraviolet, and they are good for riding on roads or trails where you need protection from bugs, as well as flying gravel, air-rifle pellets, whatever.
If you can't afford fancy name-brand glasses, see if your local safety-supply merchant has something like these. Two eyes per lifetime, maximum... make 'em last.