While progressing through the game you will receive word from Alice from time to time that there are rumors of a barn find. Once you have been notified, open up your map to find a grey circle showing the rough location of the barn. You will need to search within the grey circle until you locate the barn. Once you find it, drive up to the entrance and the achievement will unlock.
The barns are usually easy to spot and all look like this:
NOTE: The location of what barn will produce the next car will be random. You will also need to wait until you receive notification of the rumoured find, as driving to a barn beforehand will not work.
(DEG23 credits NeedToAchieve for the video guide showing the barn locations and the cars within.)