I use this one. Works as well as OEM, and much cheaper:
https://www.amazon.com/KINGDO-Surface-Adapter-Microsoft-1625/dp/B01GPFM5GA/ref=sr_1_4?s=pc&ie=UTF8&qid=1516811174&sr=1-4&keywords=surface+pro+4+charger&tag=hawk-future-20&ascsubtag=UUwpUvbUpU3730797