These are arguably the most recognizable grunge era accessory. Kurt Cobain notoriously donned this model for more than one photo shoot. You can still find them vintage from time to time on sites such as Ebay and Etsy. Or you can score them new, if you can find them in stock.
A few years ago the 6558 was reissued by Silver Lining Vintage for Opening Ceremony. Make no mistake, unlike most things associated with grunge these were always designer sunglasses. However, they were probably cheaper in the 1990s than what Opening Ceremony sells the reissue for, around $450.
If you hope to get a hold of these truly iconic shades, keep your eyes on the auctions. They’re cooler vintage, and sometimes possible to purchase at under $100.