For this achievement, including the 10 Barn Finds, you need to progress throughout the series of Road Trips in order to unlock the Barn Finds. They appear over the radio but also display a button text notification when one becomes available, just hit
depending upon your controller configuration and Assists. To supplement where all the Barn Finds are I am including the IGN wiki video which nicely shows how to find them all.
The video belongs to "Somewhat Awesome Games" originally posted on Youtube and can also be found on IGN.http://ca.ign.com/wikis/forza-horizon-2/Barn_Finds?objectid=...