Visiting my girlfriend and forgot my switch charger; can I just buy a USB-C cable from any old store and plug my switch into a usb charger?
I don't own a Switch so I don't know for sure, but what you're suggesting is the entire point of all USBs
It's actually a little more complicated than that as you want to make sure you choose something that can actually charge the Switch at an acceptable rate/charge while playing. If you're going to try to use a standard USB charger (ie, not something designed for USB-C specifically) it's going to get a little prickly.
A quick Google search led to this handy article: http://www.nintendolife.com/forums/...ch_usb-c_and_portable_battery_recommendations