OK.. to answer your question... Not sure.
I did a quick look around and I haven't seen any USB-C Y cables out there. There probably is, but they aren't easily found.
Blame Samsung.
Had they not managed to put a mini-volcano in the Note 7, there would be significantly more USB-C devices out there on the market. Accessory manufacturers build to their audience... and if there aren't many USB-C phones on the market, they won't bother to make all that much. But that's changing. The Pixels were popular, and LG, Samsung, etc. are all switching over to USB-C... just takes time.