I hate trying to source these weird interconnects. No one seems to know exactly what they do and therefore every supplier alls them something different. I looked through all of my usual vendors websites and some I don't use and I didn't find it. I also serached on Google with any combination of WiFi, GPS, Bluetooth, GSM, LTE, Antenna, Flex Cable, Interconnect...nada, nyet, nothing!
The iPhone 6 equivalent is here. I used that name as a start for the Google searching.
I'm just putting this out here so that you don't feel like it's weird not to find it. Hopefully someone will have ordered one someplace and remember where they got it.
I would contact a seller like this one or a a Chinese vendor directly and ask them (with a picture) if they can source it. These guys can generally get anything.
Please post back your results so that everyone else can benefit!