Why do you need wheel spacers? Just curious, I would imagine several Mustang businesses would have them, or ebay. Keep in mind though that they are not safe and at best you need extended wheel studs.
Maximum Motorsports has them (boxed by H & R), I would not get spacers if they are not hub-centric. MM has hub-centric spacers that are as thin as a 1/2 an inch but do require longer studs for most applications.