Geekbuying are selling four Android TV boxes based on the RK3399 , The VORKE Z3 seems best specs.
Also there is a development board by Firefly-RK3399
These may be cheaper and available now option.
https://www.geekbuying.com/search?keyword=RK3399
Linux SDK for RK3399 for Videostrong VS-RD-RK3399 also sold by Geekbuying MECOOL Development Board RK3399