Good luck, they're rare. You could try car-part.com or rearcounter.com
Here's a pic of one with the part number from the manual box I have in my truck. The splines (1"-32) are the same as some Jeep and GM 525 boxes, so you could use one of those if you can't find one. I'm using an XJ Pitman arm, part number 52000616, which has more of a drop than the Ford one. Some of them will have the square splines in the wrong spots, but you can file them down so you can mount it in the correct spot.
E37A-3590-CA