Your best bet is almost certainly off-lease corporate PCs. They will have video but you can ignore it.
If you want literally no video card at all, you probably have to build your own. Or find one that came with an actual powerful GPU and rip it out. Even a raspberry pi has HDMI.
https://www.theverge.com/2023/7/19/23800006/intel-nuc-comput...
There are so many alternatives to NUCs, it always surprises me that people think Intel NUCs are the only choice. Just lookup 'Mini PC' on Amazon. There's thousands of choices other than NUCs, and cheaper too.