Here are two alternatives.
Matias Keyboards
Check out Matias keyboards, designed for Mac. They have an extensive set of symbols printed on every key.
KB Covers
KB Covers makes molded silicone sheets that fit over every different model of keyboard that Apple makes, for MacBooks and desktops. They have one model that provides printed symbols that show many different OS X keyboard shortcuts. Their product is inexpensive and it works with the Apple-brand keyboard you already have.