Unicorn snot is apparently the go-to tie in for our release today, which comes in colors resembling the three main mares of the movie. It's actually body glitter gel, which is something I just learned exists today. Yes, I'm that good at living in a cave.
Anyway, you can get some over here once it's available. For now, they are just doing signups for the announcement once released.
Thanks to Sombra for the heads up.