It's fairly easy to set one up, assuming you have access to a Linux computer and some Linux knowledge:
1. Burn a Raspbian image to your SD card.
2. Mount the boot partition of the SD card on a Linux computer, necessary because the card uses the EXT4 filesystem which is likely challenging to make work under Windows. I run GNOME-based environments everywhere, so this happens when I insert the SD card into a connected reader.
3. cd into the directory where the boot partition is mounted. This is the directory containing config.txt.
4. Run "touch ssh". This creates an empty file named "ssh". When Raspbian finds /boot/ssh, it automatically enables the SSH server.
5. If you need wifi, place a wpa_supplicant.conf file in this same directory. I won't explain how to configure this because it's documented exhaustively elsewhere. If you're going through ethernet, I think it's sufficient to plug it into a network with a DHCP server. With the wpa_supplicant.conf in place and configured for your network, the Pi should automatically associate on boot.
6. Run "sync" to ensure that the filesystem buffers are synced to the card, then safely eject it.
Now, with the card in your Pi and the Pi powered, you can ssh to [email protected] and do the rest of the setup. I realize it isn't quite as easy as plugging in a monitor and keyboard, but it's fairly straight-forward once you've done it a few times. I have one Pi acting as my router, another running Home Assistant, and a third kicking around somewhere for another project. I have a boot/ directory containing the empty ssh file, along with a pre-configured wpa_supplicant.conf file and a config.txt with the settings needed for my HDMI receiver. I basically burn the card, copy the files, and I'm ready to boot and configure. And if one of these systems fails, or if I decide to rebuild it, I can usually flash it with confidence that it will at least come up on my network and be accessible with a default password, even if it doesn't do what I intended.