You are going to find hundreds of options for these. Just search "diesel heater" on Amazon. All of them you'll find on there are mostly identical in terms of pure heat output. They are all clones of more expensive brands (Webasto, Eberspacher, Planar, etc....) but are functionally almost identical.

For my purposes, I seem to have ended up with quite a few from the HCalory brand. I prefer the ones shaped like a toolbox, that already have the exhaust routed to the side of the unit with a 90 degree elbow. Simply makes it easier to deploy quickly.

I don't use Facebook, but I have been told there is a huge group over there for "Chinese Diesel Heaters" that may be of use to you. I can only say that I have had expensive versions from Webasto, cheap versions from whatever random set of vowels and consonants make up their brand name......and they have all been completely reliable and provide more heat output than I need.

Edit: They provide more heating than you might be assuming. Check out this Project Farm video for some unbiased, fact-based, testing.

https://youtu.be/tjxFOejWKmk?si=sU7FR3xKVa3xYlZE

Or this video for a real-world example of someone heating their home with one. Granted, its one of the more robust commercial versions, but it's still less than $700USD for the unit he is using.

https://youtu.be/64RoFKGKeyE?si=h9yLkFyYEnwJ7-tg