I've seen them offered on the local CL.
They show up on eBay too, but that still doesn't make them particularly easy to actually lay hands on. They continued to make some using dinky 5" and 9" BW CRTs into the early 2000's, but the quality of those monitors is terrible. (Not talking about screen clarity, just basic manufacturing competence.) Some of those late ones also use odd wiring, like RJ-plugs carrying video from the cameras, and I'm not sure if those units would be straightforward to adopt to normal composite input.
I confess to not knowing enough about security camera systems, but would this
cheap HF setup be capable?
It's not hard to get a cheap "modern" LCD monitor that takes composite input. For instance,
these little 8" 4x3 aspect ratio panels with a native 1024x768 resolution have VGA, HDMI, RCA composite and BNC on them, and if you look around you can get them substantially cheaper than this Amazon listing. (I think I paid $40 for mine.)
Again, the issue with these that I've found is that their built-in A/V scaler *hates* "240P" format video; they treat it like broken 480i and give you a flickery comb distortion. The same problem is present on a cheap bare LCD panel/scaler board combo I bought from AliExpress a few years ago, and I also have an older battery powered ATSC portable TV with composite inputs that likewise mangles non-interlaced input. If you're looking for a monitor that's just good enough to tell you if something "works" than, sure, these things will do that, but the quality is irredeemable if you actually want to use the thing. Which is why...
I have had terrible luck with those cheap adapters
I use one of those "cheap adapters" connected to the HDMI port of that little 8" monitor if I'm using it on the testbench, because they do a better job with 240P video.
In defense of those cheap adapters, here's a few ad-hoc shots I just took on my cluttered workbench. Here I've got one of those adapters connected to the DVI port of a 17" NEC monitor someone in the neighborhood left on the curb a few months ago. The adapter's set to output "720P" because "1080" is too high for this monitor to sync with. Here it is displaying 512x192 TRS-80 Model III-style text with a 12Mhz pixel clock generated by that breadboard in the background:
The final output stage of that circuit is literally just a couple resistors mixing sync from an AVR pin with dots output from a 74LS166 shift register and a really crude 75ohm termination, IE, exactly the sort of quality you'd expect from a 1970's computer, and I'd say it looks pretty great all things considered.
I also hooked it up to a real 1970's computer, my basket case eBay'ed-out-of-a-storage-unit D-board-with-lots-of-hacks TRS-80 Model I:
The news here isn't so great, as you can see the output is pretty noisy, but but in its defense the Model I is *notorious* for having lousy video output with extremely poor contrast and stability, and this particular example is far from the best you're going to find. And the output is perfectly usable.
(Kinda sad that my breadboard outclasses it this badly, honestly.)
Anyway, considering you can get one of these widgets for as low as $6 and most people are going to have an HDMI cable and a monitor they can use it with lying around I'd say the bang for the buck makes up for the quality. It should be more than good enough to see if the OP's Ohio Scientific C1P is at least capable of slinging letters up on a screen.
