You could order a pair of clip-on sunglasses for prescription glasses for the store where you buy your prescription eyeglasses. In most cases, they have custom magnetic clip-on sunglasses for your eyeglasses. However, some online optical store such as zenni who also sell some clip-ons but they are not customized.
A month ago, I ordered a pair of clip-on from zenni.Once received, I was impressed that they fit the exact shape of my eyeglass frames; they look and fit great.The brown tint noticeably increases contrast and the lenses do not introduce any optical distortion that I can see. Regarding the quality: I disagree with some reviewers who say the quality is only okay; my opinion is that these have a nice quality feel to them; yes they are thin, but they wouldn't really work very well as clip-ons if they weren't thin.
These provide everything I could ask for in a pair of custom clip-on sunglasses (good looks, great fit, secure attachment and great optical quality); definitely recommended - just be patient after ordering!