Key limes (Citrus × aurantiifolia) aren’t grown commercially in Hawai‘i, so they’re a specialty item. Your best bets are upscale grocers and farmers’ markets that import them from Mexico or Florida.
Vendors occasionally bring in small boxes; arrive early and ask.
Befriend a bartender or pastry chef at a restaurant known for Key lime pie (e.g., Pieology, Highway Inn). They often know which distributor delivers the freshest fruit and may sell you a few from their case.
Persian (Tahiti) limes are ubiquitous and work in most recipes; add a splash of lime zest + a drop of orange flower water to approximate the floral aroma of true Key limes.