I don't have a pattern for shams, but you really don't need one. If you have a sham, just measure and copy.
If you don't, measure the pillow and add 2 1/2 inches to each of the four sides. Your finished sham will have two inches extending past the pillow on each side. For the back, simply make two pieces that overlap, equalling the same measurement as the front. I usually overlap mine about four inches.
Lap the back two pieces on the front (make a narrow hem on the sides that won't be sewn into a seam) over laping, stitch around the edges. Turn it and iron, then stitch 2 inches from the edge all the way around.
Presto, you have a sham.
Well, it does work, though I am not sure my instructions are as clear as they should be. :)