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Here is the manufacturer and part number list from Parttarget.
This website says they sell new, rebuilt and parts kits. I didn't see prices but might be worth a shot...
Yes. For them to fall under that NSN they will all interchange. You could also rebuild the cylinder. The thread below details this process.
http://www.steelsoldiers.com/showthread.php?138637-5-Ton-Power-Steering-Assist-Cylinder-rebuild