Cheap simple amp for outdoor speakers?

So, I am currently installing speakers in my backyard, they are attached to my garage and the wires are run into the garage where I would like to put a small amp, connected only to a Airport Express for Airplay. When I go to BestBuy or Futureshop the only amps I see are full fledged receivers, much more then I need for my setup. Can anyone suggest what to use to power my speakers?