The C15 is a standard power connector, it differs from the C13 in the temperature rating. The C13 (without the notch) are only rated for 70 °C, while the C15 (with the notch) is rated for 150 °C. Since the heat exhaust in most PoE switches is the back of the power supply where the power cord connects, it can get quite warm there. Both connectors are rated at the same amperage (10A) and voltage (up to 250V).
Here is the Wikipedia article, in case anyone wants to go down that rabbit hole:
https://en.wikipedia.org/wiki/IEC_60320
You can source the cables from anywhere, we've been using Tripp Lite cables because they are available in multiple colors, so you can color code them based on what phase they are supplied from.
https://www.tripplite.com/heavy-duty-computer-power-cord-iec-320-c14-to-iec-320-c15-6-ft~P018006