Searching for "back keyhole panty" on Amazon yields some results. You can keep looking around, since I don't know if the Honeydew ones are good quality. Here are some more:

Amazon.com: Chantelle Women's Rive Gauche Tanga: Clothing

Amazon.com: Felina Women's Lace Appeal Cheeky Panty Hipster Panty: Clothing

Amazon.com: Josie by Natori Women's Cutie Lace Brief Panty with Keyhole Detail: Clothing

This one's VS, and I don't know what it's doing on Amazon, but still: Amazon.com: Victoria's Secret Satin Low Rise Bikini Panty/ Small: Clothing