If your online (live not multiplayer) when you bring up the map and it has the option to buy it, if you don't have enough tokens it will give you the option to buy some. I think you can also get them from the marketplace in the game, the one you drive to or fast travel. (not the dashboard or xbox.com)
I ended up buying the map because i only have a few hours a day to play the game and for me personally it is no fun having to drive around finding a barn. But i agree it should have been obtainable in-game. Maybe drive on every road or something.
"You see my donkey dont like people laughing, gets the impression people are laughing at him" - Master Chief GT :Shamefulanomaly