You need to measure the cylinder from just behind the head (the big part on the front) to the end of the lock. Looking at your pictures, it is most likely a 1" length, but measure it to be sure!
It is easy enough to find mortise cylinders on eBay.
http://www.ebay.com/sch/i.html?_sacat=0&_from=R40&_nkw=mortise+cylinder+key&_sop=15 will get you a start. Just make sure you are getting the correct length cylinder, as you measure in the above step. Too long, and it will bind and hit the lock cylinder on the other side. Too short, and it will not reach far enough into the mortise the lock screws into. You will most likely need to unscrew the cam (lever) on the back of the lock and move it to the new lock cylinder to ensure proper operation of the lock.
Good luck!
Gordon
Just when you finally think you have learned it all, that is when you learn that you don't know anything yet.