AliExpress has quite a few variants. Just search "USB C mini camera". They should all work fine with the underlying Linux OS, though picture quality won't be the best.
Unfortunately I am banned from Aliexpress. But I did find one with bad reviews on Wish.
Another person on the Gaming on Linux Discord has found this one on Amazon for €57. 
A bit expensive but its pretty nice option!
How did you manage to get banned from aliexpress
Storytime on getting banned from aliexpress? Lol
I don't know if it works on SteamOS, but I use my deck in Windows very, very frequently due to using it as a main computer and I had a giant stack of years worth of code cards from the SUMO/XY era I never touched. I just used something called Camo to used my phone as a webcam and got through the stack pretty quickly.
I imagine there might be something similar for SteamOS, especially since you can use a phone as a mouse and keyboard with KDE Connection.
Droidcam is a pretty decent option too for using Android phones as a webcam, that's what I've used on Linux to accomplish similar things. Just run the droidcam server on the Steam Deck and either connect to it from the phone via local IP address or over USB.
Literally only found this out the other day, great comment. I needed head tracking for beamng to complete my setup and found my phone's camera obviously outperformed my webcam. No latency over usb! Anyone wanting to know an easy way to head track with a webcam or phone should try to use droidcam with Open track. :)
That's actually something I have not thought about at all, despite having used DroidCam a lot in the past.
Asus has some tiny ones that they bundled into their gaming systems that didn't have webcams.
These?

Steam Hardware
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