Since today you can find spare batteries and a charging dock but I don't know the quality of the product. But the seller is a good one. You can trust him. I bought my yi cam with him.
Xaiomi will be releasing most of there accessories next week . A lot of stuff is on presell at the moment as some of the stuff has not left the factory . Bangood , gearbest and Geekbuying are all great sources for buying spare batteries.
How do you know that information about release of accessories for Yi, Kiku? I know there is a release date on banggood website (waterproof case), but that's not very reliable I guess
I have a contact who notifies me on such things and sells xiaomi products in the UK . He said the waterproof case will be coming out this week to supplier and be fully available next week .