|You don't need any kind of ISP to install a LAN (Local Area Network)|
Basically all you need is two or more computers, a Switch/Hub and cable to connect them and you have your LAN. If you want to connect your LAN to the Internet you plug a router into the Switch and a Modem into the router.
Some routers come with Switch, Router and Modem all built into the same box.
After all LANs existed long before the Internet did.