Hi Robert. You can make your own maps using various methods , map making software, online sites, graphic editing software( Gimp is free) etc., or you can use stuff found on the marketplace( maps , or entire modules ) here on roll20. There are also many free maps that can found online using a search engine . You can use images of maps from adventures that you own using copy/paste and screencaps. There are lot's of methods. It's difficult to share since GM's can't share their libraries. If you are a co-gm you won't have access to the transmogrifier tool.