You can purchase cheap plastic sunglasses from wholesalers at relatively little expense, and then mark them up by many times the purchase price. It is not unusual to mark them up by over 500%! They will still stand out as a bargain to customers who have left theirs at home or sat on them in the car.
There is a thriving market in (legal) replica sunglasses
CTS Wholesalers - http://www.ctswholesalers.com/ - UK 020 3004 9133 are wholesalers who sell sunglasses in bulk.