Linux

For a lot of purposes, an old slow system is great. Craigslist (if you're in a country that has it, otherwise use your local equivalent) is a great place to find super cheap old equipment to build server infrastructure from. Thrift shops too, I regularly find old UPSs that just need a new lawnmower battery chucked in to work at my local thrift shop, they charge like $10 for em. Usually found near power strips.