So I've tended to assume, possibly stupidly, that if a device relies on a USB power connection at all then any old AC->USB power adapter is fine to use for it and substitutable for a computer USB port ...
Update 12/05/18: Apple now sells an 18W USB-C power adapter for $26Remove non-product link. To use it with your iPhone, you still need to purchase a USB-C to Lightning cable. We haven’t tested it yet, ...