I would start by getting a Kill-A-Watt (or generic) so you can measure power draw (under artificial load). The price between a 600W UPS and a 1000W UPS can be dramatic. When I hear "small server" I think of an R210ii / similar platform that uses less wattage than a old fashioned light bulb.
If you’re patient and can self-support, your local non-profits and their online stores may be useful.
It’s rare to see big gear NIB, but it does happen; have seen rack mount Cyberpower, 10 outlet, hi-watt units, IOB going for around $200+ in the last 6 months. Picked up a previous generation, 5 disk NAS for a tenth of its retail price, used.
You’re north of my border so my sites won’t work for you, but seriously, look into any Goodwill/Salvation Army type organizations, and don’t neglect local, they can be rewarding too.
This is the best way, get one cheap or even free with dead batteries. Then just replace the batteries on the cheap and you've got yourself a UPS that was probably a few hundred or more $$ for the cost of maybe $100 in batteries.