Why do they need fo be open source?

Broadcast a second wifi network that blocks all outgoing traffic(i.e. no ability to access the internet or devices on the rest of your network). Use any ONVIF network camera with wifi (hikvision or dahua make good cheap cameras), put them on that locked down wifi, and configure your network so that your NVR can reach the cameras.

Ideally, you would use wired PoE cameras (and either fully air gap that network or separate it like my WiFi recommendation), but if you’ve gotta go with WiFi, pick one that’s designed for use with an on-prem nvr and not a cloud service.

Have a look at Open IPC and Yi-hack (link is to one specific family of their cameras, but there are hacks available for other non-allwinmer models too).

I believe you can also hook up a raspberry pi (with a camera module) to Motion Eye wirelessly.

The easiest thing for security (though not necessarily own source) would be to find a WiFi cam that offers offline or local access and confining it to it’s own subnet that doesn’t have access to the internet. I think you can do this with reolinks, but double check.

fwiw over used reolinks cameras and have been satisfied with them. I have them integrated with Home Assistant, just use a firewall rule to restrict them from reaching the Internet as I’m paranoid about that stuff. They’re not open source but they do function 100% offline without any issues.

I heard some people confine cameras to a seperate virtual network which can’t talk to the internet.

I don’t have any specific camera or manufacturer to recommend. But Frigate is a nice peace of software. Maybe their community or the Home Assistant users have some advise.

If you like coding and soldering, you can buy an ESP32-S3 camera and do the entire firmware yourself. I don’t think they’re as good as off-the-shelf cameras, though. And I think most of them come with a cheaper OV2620 camera, which is more close to potato camera quality. So if you go that route, spend a few $$ extra to get one of the better camera sensors.

Open source as in open hardware? Not likely, but I could be wrong.