Every node connects to a central network device. like a hub, switch, or computer.
Generally I'd recommend not buying one online, tends to be cheaper from a shop - and better used, although their are some risks with 2nd hand devices, I think the big price cut it worth it.
If you did want to buy online, maybe Amazon or Ebay?
Someone can install an operating system by disk or removable storage media.
Disk being floppy, tape or commonly a DVD.
Removable storage being a USBdrive.