Do places such as Gamestop or Best Buy sell Xbox One Power Cables?

They currently do not, no.

http://myservice.xbox.com/ does though

Gamestop is not the only place to sell and trade games.

I think buying games online would be the best way to go, but I doubt that everyone would just stop buying physical games.

nope, not the consoles but dont know if I dreamt or someone told me just to mess with me but sometimes they sell refubrished consoles as new ones, at least way back when they didnt have the slim, also due to availability I would rather buy it any other place
before I got my newest 360 everytime I went to gamestop they never had them on stock

also yes I sometimes buy games at gamestop but only like the non best selling ones

So there are options, but not directly from Microsoft. The Xbox One S and X have an internal power supply, so you can buy a replacement cord, but for the Original Xbox One they no longer service/replace/sell the power supply for that model.

This article has some information on the best place to buy a power supply for your Xbox One, but please make note of purchases and regions in terms of voltage and wattage.

The only other way would be to buy a prepaid card in a store or some places sell the codes digitially (I know Gamestop and Amazon do).