You can find codes through specialized retailers and community marketplaces.

Buying is a popular way to build a high-quality movie library for a fraction of the cost of buying directly from the Apple TV app . These codes are typically sourced from physical Blu-ray or 4K Ultra HD disc bundles and can be redeemed for a permanent digital copy in your account. Where to Buy iTunes Digital Codes

What are the best sites to buy digital codes? : r/MoviesAnywhere