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I knew this day would come.
So here's the thing, 6th gen sprites aren't included with Essentials because as of now there aren't really any good 6th free-use sprites. Smogon/Showdown are making their own, and you can ask them for permission to use theirs with credit. However, they denied us when we asked, and I can't promise that they won't deny other games.
There are actually a number of decent quality front sprites out there now that are free use, the most popular of which is surely Noscium's set. However, at least when I last checked, the only decent back sprites floating around belong to Smogon (and even those leave a lot to be desired).
However, even those are 5th gen styled (as are Reborn's). If you want to use 4th gen styled back sprites for 6th gen, there's a topic on Pokecommunity where people are creating them. I personally wouldn't use those, however, because I do not recall being impressed with their quality when I last saw them (perhaps they've been bettered, since).
Anyway, that brings us to Reborn. Not to toot our own horn or anything, but assuming you're okay with 5th gen styled back sprites, Reborn probably has the highest quality sprites out there (Smogon will probably end up better once they finish, but a lot of their stuff is in a rough state right now). I've gone over every single one of them myself, and while I'm not 100% satisfied with everything, they are better than anything else I've seen. You can find them in the 10.5 Download, but you'll be on your own for shinies.
HOWEVER, usage of these comes with two caveats.
Firstly, you have a long list of spriters that need to be credited for using these. I'll post it here, but it's also in the 10.5 Readme. I was pretty diligent about only using work from artists who granted permission-to-use as long as they receive credit.
There is one exception, however, which is the second issue. Our Greninja front-sprite is made by a different spriter who specifically requests that s/he be asked for permission to use it. Xe granted it to us, but that sprite isn't mine to give away. You can contact them on deviantart under the name Vale98PM or find a different Greninja front sprite.
Here's the list of spriters to credit if you use ours.
+ Whatever you decide to do about Greninja.