Are there USB-A to USB-C adapters for the Early 2015 MacBook
Simple answer - yes.

Apple has also announced an USB-C to USB connector for £15 in the UK store or $19 in the US store.


USB-C to USB (UK Store)
USB-C to USB (US Store)


It too isn't shipping yet, so it's hard to know if this is a custom chip and design or any old "connect the wires" adapter will do the trick. However, my guess is that it's just a simple adapter and there's no licensing like the lightning port requires/allows.

The specifications are that the port is just USB 3.1 Gen 1 which means that pretty much any USB-A device in existance will interoperate with the new MacBook.