Researchers have built server prototypes that re-use old components

I find a lot of 10 year old Dell PowerEdge servers for basically free these days, some loaded with 128gb+ RAM. They work perfectly well with TrueNAS, pfSense, or even more powerful stuff. If you dont need a thousand cores, I always suggest them to people. Otherwise they end up in the dump..

Where are you finding these systems?

You'll find them readily available on Ebay, but there are also multiple companies that specialize in refurbishing servers (which usually will allow you to configure your actual needs - but this will be slightly more expensive in my experience).

Someone just moving decommissioned servers from a data center to new users without doing anything with the equipment in between allows you to find decent deals if you're looking for something to put in a rack.

Be aware that rack servers are usually rather power hungry, so they might be expensive to run over time.

I found eBay to be strictly an USA thing, plus Canada at best for those who are OK with driving for half a day, or even two.

I have the app installed and the shipping costs to Eastern Europe where I live often surpass 50% of the price of the tech itself.

I'd love to reuse. A lot of us out there who are still oldschool-ish and can work miracles with older tech. But I am not about to spend the same money I'd spend on simply building a PC with EATX case and the ability to shove 12 HDDs in there. I'd still end up spending more on the local market, mind you, but we're looking at 10-15% maximum and I don't find that a worthy difference to wait 3 weeks for an older server, especially with a very high likelihood of also having to pay 20% of the value of it to customs.

You'll find vendors in Germany, Spain, etc. within the EU. Not much I can say about customs - so you might want to check local recyclers. There's usually some sort of recycling program for old hardware that gets cleaned out and re-sold, but it'll all depend on your country's incentives.

Whether it's worth it will depend on what you're looking for.