I'm not in Europe, but I've had good results from a site called "server part deals" ( no spaces ). It's a "see oh em" site, not sure what the rules are here about posting links.

I also have "hard disk direct" ( no spaces ) bookmarked, another "see oh em" site. They have motherboards, etc, while the first site there seems to only have disk drives, now.

The default answer these days seems to be ebay and facebook, sadly. Found that out last week when asking about retro computer hardware.

Not sure which country you’re from, but if you’re Dutch you can check Tweakers:

If you have a big order, I’m sure most sellers speak English and might be willing to ship outside of NL (at your risk).

If it is a small country, hang out at wherever the e-waste drop off is, and offer to accept the machines in liu (sp?) of the probably greenwashed megacorporation. Most folks will be willing to give you their stuff if they know you'll actually USE it and you ask nicely.
eBay is probably the most common place.
I live in Texas, and buy from:

1. Local people who get them from auctions.

2. eBay

3. Web sites run by companies selling the PCs.

4. Amazon

Facebook marketplace
eBay and Amazon Marketplace is where I normally go