1st, do not power the fan from the pi if possible to avoid any underpower headaches. It should be fine.

Your best bet is to just power it at 5v using a standard USB wall wart and a sacrificial USB cable (old 2.0 ones work best). Strip the USB cable and expose the positive and negative wires and hook them up to the positive and negative fan pins (identify these using a 4pin fan header diagram). You can either try just shoving the tips of the USB cable into the fan header using electrical tape as a quick bodge, or cut and strip the fan cables and twist them together.

Gotcha! Thanks for the directions! Any reason why I shouldn’t power it with 12V, since it says 12 on the fan…?