How can I migrate my data from my Macbook Pro 2010 to a Macbook 12-inch 2016? How can I connect a Macbook 12-inch 2016 to a Macbook Pro 2010 in target disk mode to migrate my data with Migration Assitant? The Macbook has a USB-C port and the Macbook Pro 2010 has Firewire 800 and USB 2.0.

I couldn't find an adapter or cable for this.
Your best bet is to use an ethernet (network) cable. At gigabit speeds it should be pretty fast.

The 2010 Macbook Pro has an ethernet port, and since the 2016 Macbook has nothing but USB-C you will have to go with an adapter.

This one was the first result in my search engine of choice. There are many others. And I would imagine that if you buy the adapter from a store with a good return policy and use care in unboxing and using the adapter, it might be easily returnable, if you only use it that one time.

Also, it has been a while since I have had to do this but Migration Assistant may work over WiFi if both are connected to the same WiFi network. That would be kinda slow but should do the trick if Migration Assistant works with WiFi