How are these mounted to the deck? I don't see where a separate mounting kit can be purchased.

These mount directly to the post, they don’t mount to the deck itself. Your post is what gives it structure (this sleeve is really just for looks and protects the post). When installing these, make sure the post is secure. These just slip over it. In reality, it is only when you screw the railing brackets into it does it even get secured to the post. If you’d posts are good, they work really well and look nice.

Hello, and thank you for your interest in Trex railing! This product is for our 4x4 hollow post sleeve. Since this is not a structural post in itself it is pre-shimmed and designed to fit over a 4x4 pressure-treated post or equivalent post mount. Thanks, Tori - Trex Customer Care Team