How to get custom maps?

I was wondering how to get custom maps for halo 3(or tree if you are Irish) because you can’t get it from bungie.net and i don’t know where else i could get them

Download it from people fileshare

Bungie.net still hosts file sharing for the games but it is no longer updated. If you can find a link to their Bungie file share, you can find the file and get it from there.