LifeProof headphone adaptors are available on the LifeProof website. In 2015, an adaptor cost $15.99. Amazon and Newegg also sell many adaptors for waterproof cases, but are generic models.
The LifeProof website has two adaptors available, one for the iPhone 6 and one for earlier models, Android phones and iPads. All the adaptors work for the Fre or Nuud cases.
Amazon and Newegg sell many generic adaptors. The cords are cheaper in price, and are offered as a single cord or in sets of three.
Major retail stores only offer entire cases or larger accessories, such as belt clips or running bands.