Thank you for your patience as we tracked down some more information for you. Unfortunately, Native Eyewear does not offer replacement parts for purchase, so your only course of action would be to submit your sunglasses as a warranty claim and see what your options are via that process.
Part of the issue you are experiencing with their website is due to a transition with Native Eyewear's parent company to Luxottica via a merger that happened in March. You can follow this link to the Luxottica warranty, repairs, and replacement parts section. They do list Native as a brand there. If you click through it asks you to register/log in to continue the process. Additionally, you can call 1-888-77NATIVE to begin a warranty claim on your sunglasses.
Hopefully this helps, thank you again for your patience!