https://www.fanaticanimestore.com is where I just ordered. There's a few other places like toypanic.com and such that have them up for preorder as well.
I have some bad news. I contacted LB Studios on Facebook for an update and they've told me they are no longer producing the helmet. They've scrapped plans to make it.
I pre-ordered mine through Fanatic Anime Store. Upon relaying that information to them, they've confirmed it for themselves and immediately sent me a refund for my deposit. They SHOULD already be sending refunds to any one else who ordered. If you don't receive one soon, I would contact them.
If you've ordered from somewhere else, definitely contact the seller. Feel free to forward my screenshot of LB Studios' message for proof.