https://geizhals.de is a German search website that allows you to very narrowly specify all kinds of things.
Searching for a mainboard with minimum 4x SATA ports and 64GB RAM support in mini ITX format? Easy peasy.
For example, take a look at the laptop category and its filters: https://geizhals.de/?cat=nb
I wish every e-commerce website was built like this.
Here are some filters I find interesting to use.
You can filter HDDs by recording technology: https://geizhals.de/?cat=hde7s&xf=8457_Conventional+Magnetic...
Similarly, it's possible to specify the storage cell type for SSDs. https://geizhals.de/?cat=hdssd&xf=16325_3
For RAM you can specify which memory rank you want and the actual ns operation times for most primary ram timings: https://geizhals.eu/?cat=ramddr3&xf=15554_10.6%7E439_dual
Sadly I don't buy through them very often, as they only show stores for Germany, Austria, Poland and the UK.
My only criticism of geizhals is that you can't see items that aren't for sale anyonger unless you search for their names directly. For example you can't use geizhals to get a list of 5k monitors models sold in the last years (most of them are no longer on sale).
Something like a "discovery" view would be nice where you could specify your requirements beforehand and then look for e.g. compatible parts that could be interesting.
It reminds me of the GSMArena's Phone Finder (https://www.gsmarena.com/search.php3?) which also allows specifying very narrow search criteria. But it is specific to phones and tablets only.