You should be able to get them through Mustang/performace supply houses.
I made the ones that are on my '68 out of 1½" thick, by 2" wide, by 4" long solid blocks of steel. I drilled a hole all the way through the center of the block and threaded the hole half way through to screw an Allen head bolt into the block. The head of the Allen bolt serves as the locating pin that fits inside the hole in the spring/shock plate.
On the opposit side of the block, using the original hole I drilled for the Allen bolt, I used a bigger bit to open the hole up to the diameter of the pin on the leaf springs.
I took a larger diameter drill bit, ~1", and made a large hole through the blocks on either side of the locating pins to help lighten the blocks up.
You will have to have longer U-bolts to compensate for the greater distance the blocks will move the rear end housing away from the spring plates.
http://img166.imageshack.us/img166/2741/loweringblocksh0.jpg
www.ultrastang.com