I have a motherboard, that requires a DC input. The manual has this:
I'm trying to find a power brick, but all bricks I can find usually have +12V/-12V on pins or shell and no ground.
Can I use them? Or can I modify them?
This is exactly what you need, "-12V" goes to "GND", and "+12V" goes to "DC_IN". You don't need to modify anything if your have the pinout shown in your second picture. This is a common wiring for "Power-DIN" connector. Keep in mind however, that pinout of power supplies sometimes use different pin assignment.