I buy all of my UPSs refurbished from www.refurbups.com
these days. They have a wide variety of SmartUPS models on this page
. I like them because they sell new batteries with all of their refurbished UPSs.
Prices vary slightly, but right now they have a refurbished APC SmartUPS 1000 for $120. Their stock varies, tough, and they often have SmartUPS 700 models for well under $100! Shipping isn't cheap, but is reasonable since UPSs are VERY heavy! RefurbUPS.com has newer models with USB connections for communicating with a computer, too, but you don't need this for an aquarium and so can buy the older models without USB and save a fair bit.
I think that UPSs without sine-wave output can damage electric motors, but in any case they don't work well. I tried it years ago, and my powerhears made awful noises and didn't work when under the UPS's power. They weren't damaged, thankfully, but I only tried to power them via the UPS (a nice APC BackUPS Pro 1100 model) for a minute or two (as a test).