Ebay, you need to figure out what is an old height tailstock and what is a new height, they changed the swing about the time they moved from round dial to square dial machines. You might want to check out the Monarch board at PM, lots of EE info. I would say a tailstock in good shape with bore that is not scored could bring 200-400 bucks privately, more from a dealer. You will most likely need to shim or scrape your purchased one to sit at correct height.
I live about 10 miles from Sidney, Ohio where the 10EE is made and rebuilt. I talked to the service manager a while back and he said that used parts for a 10EE are abundant, but I'm not sure about the "fair price" part of your question.