I've got a pair from Optilabs which were great, but the prescription is now out-of-date. For the last year or so I've been riding in my regular glasses without any elastic bands or anything. The only issue is that above about 35mph my eyes start watering a bit. On the front of a tandem that's not great, but if you're (a) mountain biking and (b) just beginning, that probably won't be a problem. Lenses these days are pretty much indestructible, so the old issues we used to have with glass lenses cracking have gone away.
My advice would be simply to wear your ordinary glasses until you think you really need new ones, then look at Optilabs or similar.
