Anywhere to buy premade “analog TV field station” / analog channel generator boxes?

I’m putting together a small analog TV field station setup and normally I’d just build the box myself, but I’m pretty short on time right now. I know the usual approach with modulators and media players and I’ll probably go that route if I have to.

What I’m trying to find out is whether anyone sells premade field station boxes where the hardware side is already assembled and you just load your own video onto it later.

Basically something already built that outputs NTSC RF so a CRT can tune through channels, but where I can still swap or add my own content afterward.

Curious if anyone here knows of people selling these, small builders doing them, or places that occasionally list them. Even limited run hobby hardware or custom builders would be useful to know about.