There's an outfit out of Kaufman, Texas, I believe they are called Premier, or G&S Diesel Service, that is always selling rebuilts on Craigslist and Ebay.
Their prices aren't too out of line, but I can't say that I have actually seen their work.
Here's a link to an ad of theirs.
COMMONRAIL CUMMINS 5.9 LONGBLOCK
Mark.