For your purposes check a NI USB 6009 Data aquisition card. It's digital output lined can provide exactly this same voltage level that you need. The maximum output current seems to be a little lower and its about 8,5-9 mA . USB 6009 is very convinient and simple in use, but try to use NI DAQ mx version 8.0 or later to cooperate with it.
Maybe this is a silly answer due to my only theoretical knowledge of SCXI, anyway...
If your SCXI chassis is connected to PC via a multifunction DAQ board you should have access to digital and timing lines of this board, supposingly via a scxi-1180 or better scxi-1181 module.If this is true, you could use a counter of your daq board to generate the square wave signal you need, but you will need to add a transistor in output since counter lines are not capable of such a current as you require. This transistor and a couple of external components could be housed on the breadboard part of 1181 module.
I hope this can help you a little...