What can I use for sunglasses when I wear prescription glasses?

PIggy back glasses. They make large sunglasses that fit over your lenses or you can use clip-ons or get prescription sunglasses.
Clip ons. If you wear prescription glasses you have a couple of choices other than prescription sunglasses. First and best performing are magnetic or clip on shades that fit in front of your regular glasses. Key drawbacks may be glare and they are heavier. You can also wear "fit overs" which are large sunglasses designed to go over your regular lenses. See your optician for suggestions.