My fan came with a 3-pin connector and a little extension cord to piggyback on a Molex if necessary. It seems that the only kind of power connector my PSU has is the ATX 1.3 (i.e. SATA), and there's only one 3-pin on the mobo which is being used by the CPU heatsink fan.
How can I supply power to my fan?
[Edit] Thank you to @Moab! To summarize my use of their solution below: I'm plugging one of my PSU's SATA connectors into this SATA-to-Molex adapter, which plugs into my fan's included 3-pin-to-Molex-tee extension cord, which plugs into this Molex-to-SATA adapter, which plugs into the device that was originally supplied directly by the above-mentioned SATA connector. (The PSU had no spare SATA connectors, either.)