You can purchase Chocolonely at Natural Grocers and Sprouts in the US, if they're in your area. It's the best quality chocolate I've ever purchased that didn't come from a chocolate shop, highly recommend it.

Coco is sold on world markets as a commodity good. Suppliers can't know what country the beans originated. To get guaranteed slave-free chocolate you effectively have to create your own logistics for that specific purpose. Any chocolate producers that haven't done that will have some amount of slave-produced coco mixed into their supply chain.