Transformer Oil.
I used to use it in repairing submersible pumps. These use it for cooling similar to transformers, but being submersible, if water leaked into the oil the motor would quickly fail. We would need to replace o-rings and seals and re-fill.
We bought our oil from a local industrial oil supplier, but minimum quantity was a 50 gal. drum. Look for someone who repairs submersible sewage pumps. They probably have a drum handy.
How big is this UPS? I'm not sure I've seen one big enough to warrant an oil filled transformer.