It takes a 12mm glass lens that can be bought from Surplus Shed (http://www.surplusshed.com/) for $4 - part number L4471. That was the cheapest place I could find for glass lenses.
The lens drops into a recess in the holder and is held in place by a tiny bit of superglue or similar. if you get glue on the lens, it can be cleaned off with nail polish remover or acetone. The lens I used is plano-convex (one side is flat). When inserted in the holder, the flat side of the lens faces the phone.
I also should note that you need to remove the iPhone's case, if it has one, to use this.