Hi,
I'm looking for a photointerrupter that can be powered off the 5v regulated bus and has an output of a digital HIGH or LOW.
Then you either need one with a Schmitt-trigger (more expensive) or add a Schmitt-trigger yourself (more work).
You can have slotted optocouplers (SOCs), which they're also called. with the slot up to 4" wide, so 2mm is no problem, but if you need one with a Schmitt-trigger, it narrows down the selection of course.
If you can find a way to use a regular SOC, price and availability gets more to your advantage - if it's for using with a µcontroller, most have a comparator input that could be used.