You can get rebuilt ones from Pelican:
http://www.pelicanparts.com/catalog/shopcart/9144/POR_9144_fuelij_pg1.htm#item8
If you're looking for used ones, I
think any of the 1.7 ones will work, but the "right" one is from mid-71 through the end of 72. Any 72, or any 71 that has the idle mixture control knob on the ECU should be the correct part.
Why do you think you need a new ECU? Does yours have a large hole in it? Are the injectors turning on and not turning off? If the symptoms aren't one of those two, then it is very likely that your ECU is fine. Check Kjell Nelin's D-jet Guide
http://www.914fan.net/djet.html for lots and lots and lots of info on your FI system.
--DD