The power supply that comes with the Simple Metal is rated for 12 volts at 6 Amps with a 2.1 mm x 5.5 mm barrel connector. It's also the same barrel connector used in 12v LED lighting. The connector is nearly universal on small 12 volt power supplies.

Here is one sold on Amazon, but you don't have to buy this specific one. If you search for a "12v 6a power supply" you shouldn't have any trouble finding one from just about any online retailer.

https://www.amazon.com/100-240V-Switching-Supply-Adapter-Flexible/dp/B015G8E3X0

Alternatively, if you have a relatively new (manufactured in the last five or six years) ATX power supply laying around that has a six pin PCI-E connector. (the same connector that plugs into a video card) You can use that to power your Simple Metal. (may need to short together the black and green wires on the 24-pin connector to turn the power supply on) I use one with my Simple Metal and it works great! Also provides plenty of power for my heated bed.