Prescription (Rx) sunglasses (including Serengeti) can be ordered online from the states. I used
http://www.opticsplanet.net for my Serengeti Sangro Rx sunglasses. Great specs and the ordering was very simple (just took a couple of weeks to arrive). The only downside is that the prescription lenses are only available as glass. You can end up with some hefty plates of glass sitting on your nose if your prescription is on the strong side. Sunny day, hatch closed and latched, slippy nose, heavy specs. With a headset clamping them they're OK.