Maps and Treasure Maps can be found throughout the different Acts and Levels in the game, being found at the various Trading Posts and from the different Traders encountered in the game.
There is an informal area coverage for each of the Acts, as each act specifically covers in terms of game play, missions, quests, and the like, and each physical area in the game has within it its own group of special objects - and collectibles - that are there for the player to find.
Act I - Banapur Village
When the player arrives at the village that serves as the hub for Act I they will find two Trading Posts one of the traditional sort that is manned by a woman who ostensibly owns the Post, and a second in the form of a gun cabinet in the Communal Sage House.
The Traders offer a wide variety of weapons, ammunition, and kit, sell resources you can use, and also have in their inventory a selection of Maps/Treasure Maps that cover the areas in which the Act takes place.
This section covers the Maps for Act I, which are found at the Trading Post in the village of Banapur; the list of available maps includes:
It is actually a good idea to go ahead and start to acquire these early on in your adventure for a number of reasons - first, having them will allow you to get items from the maps that are close to where you are going anyway for quests; second they encourage you to explore the region; and third, they can give you significant wealth and help you earn XP and as a result, Skills.
So basically you have your four different map types that cover the four different regions that are part of Act I.
The full set of maps is not always available right off as some must be unlocked by game play or specific actions.
When you can afford to obtain the maps you really should do so, treating them as sets for each area and making significant efforts to clear each area of the different icons once purchased... Doing this actually significantly improves your game in terms of the capabilities and wealth of your character - that should go without saying!