Sourcing Shimano Di2 6770 Parts Can anyone tell me where I can source a Shimano Di2 Ultegra 6770 rear derailleur since shimano no longer has replacements and left their customers high and dry?
If they don't sell/manufacturer them anymore, then your best bet might be the used market, such as Ebay.  This article has some interesting information.  It seems like it's possible to mix 6770 and 6870 parts if you haven't updated your firmware. Otherwise it says something about using 6870 derailleurs with 6770 levers. Which I think means you could upgrade your entire drivetrain to 11 speed, without changing the levers, which would make the upgrade a bit cheaper. You could probably keep your crankset the same as well, as I'm pretty sure that it would be compatible.  

I don't have Di2 and I'm just going by what I've been able to find on the internet.  Do some more research. There's probably a good solution that doesn't require upgrading the entire groupset. I read a few other other articles while searching around, and it seems like Shimano did a pretty big disservice by dropping the 6770 product line.