There are some front sprites available, usually, but they aren't always up to quality. Unfortunately, most sprite artists aren't interested in making back sprites; those are the real problem. To tell you the truth, the Mega back sprites we do have were made courtesy of a generous user named Julian who was interested in helping the game out, but she hasn't been around lately (she was also responsible for that beauty we know as the PULSE Tangrowth).
I only did quality touch-ups on all of that, but to tell the truth I'm still pretty burnt out after spriting the 6th gen Pokemon, since I did make a number of those sprites from scratch.
I'll get over it.
Any canon Megas should be included in Reborn.