Where does the power supply go in an SR Latch?
Of course there's an external power supply.  Take a look at a NOR gate chip (say, 74HC02) and you'll see it there clear as day:


Logic symbols purely show the logic flow of a circuit, not the power flow. Many schematic capture programs will take the power connections as read and automatically connect them to the right nets for you.

Internally a typical CMOS NOR gate is made up of MOSFETs which switch the power coming into the chip in the right way:



You can see there how the output can be switched either to provide power from V+ or to sink power down to GND depending on how the inputs control the MOSFETs.