Suggestions for how to collect codes/movies?

With streaming services shifting to running ads (and me not willing to pay more for ad free), we can’t download movies on the streaming apps anymore. When we travel, we set up an iPad for our kids to watch a movie. It entertains them and saves our sanity. However, now our only option seems to be Movies Anywhere. We only have two movies so far. I’ve been following this sub for a little bit and just recently saw a post where people were commenting how many movies they’ve collected. It was impressive but I’m curious how people get that many movies.

What are your tips/suggestions for collecting movies other than buying full price for them? Thrift stores? Do the codes exist somewhere else? I don’t need a ton but would love to build a solid collection for them when we travel. I’m just not sure where to start!