I was looking at these guys as an optical solution (assuming there is a place to mount them):
Digikey# 425-1954-5-ND. Even though they take a special connector (it uses the Tyco Electronics
292133-3 connector on the module, so the
173977-3 would be a suitable choice for connecting to it), it is still much cheaper than any other pre-built optical solution (at least from Digikey)
Sharp Datasheet (PDF Warning):
http://sharp-world.com/products/device/lineup/data/pdf/datasheet/gp1a75e_e.pdf