Good luck finding a decent one. Can't remember the last time I saw one, but I'm in the midwest and they all rusted out by about 1985. We owned one around 1980, and like the above post said, the 4cyl was severly underpowered, as was the V8 for the matter. They did run good, just prone to rust.
The Mustang II has never really caught on with enthusiasts as much as the earlier models - it's really not a bad car at all - I would recommend that if you get a four cylinder model, make sure it is a standard shift because the automatics are really numb in the acceleration department.
I bought my daughter a Mustang II fastback with the four cylinder and she loved it - personally, I think I could have outrun that car with a decent pair of Nike's on.
However, she consistently got over 20 MPG in and around town - I think a standard might have even done better than that.
it's 30 kyears old, so it's going to be hard to find a nice one. Your best shot is http://ebaymotors.com set up a custom search with email notices. when you find one don't blindly send money. go to the car and buy it...
they didn't have o.d. so the mileage suffers, 23-27
I had a 65 6cyl stick that got 25 on the freeway doing 75...
I don't where you are at, but I would check with your local Mustang club. You can search the net to find them. Another good source is the Mustang Club of America. They have classified ads. Good luck the Mustang II never did become as popular as the early models.
The early Mustangs are good cars but i would stay away from it. I know i sound like a judge but stick with the dodge chargers or challengers theres some good hemi. You can go off ebay and get a mustang if you really want one.