Just about any opto isolator will do for this, provided it has a reasonable current transfer ratio at low currents. The cheapest on that my usual supplier has is http://uk.farnell.com/everlight/el1112-ta-vg/optocoupler-tr-o-p-sop-5/dp/1852690, which looks just fine, assuming you will be making a pcb and are happy to use SMD components.