Where do i get halo 3 maps required for online?

[drensle]

i just got live, so i went to do a big team battle but a message came up saying one or more players do not have the maps required to play in this playlist.(only playlists i can use are social slayer and basic training) does any1 know where i can get these maps or wut map packs contain them?

 

[some one]

when you first start the game, there should be a menu screen where you can pick between campaign, matchmaking, custom games, forge and theater. press start on your controller and scroll down to marketplace. there should be some map packs that are free. Good Luck!