I buy them from $1 store, I can get five at a time and toss any ones that don't work out.
OP, have you been diagnosed by an optician as "20/20", or do you just have very good vision? If you are beginning to develop refractive troubles, the slight errors caused by stylish sunglasses (bubble-eyed or curved) can immediately cause your strong accommodative eye muscles (which are keeping your vision good despite encroaching errors like astigmatism) to have to go into overdrive, causing headaches. Go see an optometrist for a current checkup, and bring your sunglasses, see if (s)he will let you try a Snellen test with them on as well as the real bare-eyes diagnostic session.
(Or are we getting into medical advice here? Dang, two warnings in one week!)