I think the Landsat archive (accessed through the USGS EarthExplorer) would be a great place to start. They have images going back to the 80s as well as new Landsat-8 coverage of much of the US (perhaps even all of it). The data will be in a format that can be processed in ENVI (geotiff with metadata files). Old air photos can also be used but the images will likely need to be scanned and they often only have true color (RGB) information. They would also need to be registered to a projection and then to each other. With Landsat, you have a NIR band that is particularly useful for mapping vegetation. You would likely want to first calibrate and atmospherically correct the images to reflectance, then use the Image Registration workflow to align the images to each other. ENVI has Change Detection tools that could be used to find the changes in vegetation between two images that should be helpful for your task.