Right now you can pick up Aukey's 3 pack of USB-C converters for just $6 at Amazon with coupon code AUK3USBC. The package comes with two converters to turn Micro-USB cables into USB-C and one to turn a regular USB-A cable into USB-C. Odds are you'll have a much easier time finding one of those two cables when you need some extra charge than you are to find a USB-C cable laying around, so this makes those cables useful to you. When you break it down to just $2 per adapter, you really can't go wrong with this deal.
Even if you've already invested in a few USB-C cables, these are still great to keep around. Keep one in your pocket at all times, just in case your friends don't have the right cable for you and you need some extra battery life. Don't forget to use coupon code AUK3USBC for the full savings!