I would think that perhaps the best place to look for replacement battery would be to Tesla's Powerwall2. They are using the new 2170 cells that they are making in Nevada. At $5,500.00 for 14 kWh, I haven't found anything less expensive by far. I have not been able to find out how they are configured though. What is the DC voltage of the package??
If one is paying for labor to custom build a pack (or cells) and using quality 18650's to do it. It will most likely cost more than Mitsu replacement cost. and the Mitsu pack would presumably have a warranty. I think "future" is the key word here.