If you can't be bothered finding them and have money to burn you can just buy them online.
Is there a place that actually sells muscle cars online? I know about the legendary cars and the last site under the transport tab, but that place didn't have a category for them. I've been trying to pick up 2 specific ones to fill out my garage.
Look at the map,look at the roads and see if similar,if so mark it and search around there for a car with a stripe on it,look up and down around places,pictures also help.Its one of my favorite cause rockstar doesn't hold your hand for it,gets you better used to the map