Do you have access to the inside of your case, and the standard 4-pin Molex connections instead? 2 of those wires are ground, one is +5v, one is +12v.

If you want to go to the wall, then as someone else already pointed out start looking at old wall-warts and find one that's 12V, maybe you'll need a coaxial pin adapter if you don't want to cut the wires on the adapter. You can find this sort of thing cheap as heck on Amazon too, just need to find the right size. e.g. https://www.amazon.com/ZRAMO-Terminal-Adapter-Connector-Security/dp/B06XXGH1HZ/ref=sr_1_18?dchild=1&keywords=12v+coaxial+terminal&qid=1594230456&sr=8-18

Example of a 12V wall wart with the 2-wire adapter: https://www.amazon.com/100-240V-Transformers-Switching-Applications-Connectors/dp/B077PW5JC3/ref=sr_1_3?crid=2W5P10RGEWKHO&dchild=1&keywords=12v+wall+wart&qid=1594230606&sprefix=12v+wall+%2Caps%2C193&sr=8-3

p.s. neither link is a "buy from them!" recommendation - I didn't research the seller more just did a basic amazon search and linked a likely quick example.