Losing USB connection

[Arnd]

The plug going into the motor shield. I need a supply which supplies >= 12V, 5A, correct?

What power supply for arduino

If you order a supply from one of the usual suspects get a spare or two; never know when another project will come along. With a power cord and DC cable with the proper end you can use one of those perforated metal switching supplies, which will give you more options. They’ll have exposed connections so take the usual protections against contacting mains voltages. Random example: