Power Macintosh 5400/200 - looking for a keyboard, mouse, and power cord

Any ADB keyboard or mouse would do. I suggest eBay. As for the power cable, they're just regular computer power cords. You might have a box of them around. If not, you can barrow one from your TV, monitor, computer, etc.

 

That stands for Apple Desktop Bus. It is the name of the connector used on mice, keyboards, and other simple peripherals on Macs of that time. It uses the same physical connector as S-Video, but they are not electronically compatible. It's almost like older PS/2 mice and keyboards. Note: ADB is supposed to be hot swappable like USB and FireWire, but it should never be hot swapped. Doing so can lead to a burnt out logicboard. Always power off the Mac before plugging or unplugging an ADB device.