All UPS'es (unless things have changed) use sealed lead-acid batteries.

Not the same battery chemistry, so much reduced (if nearly zero) fire risk from a UPS.

There are some out there, but not at the commercial/industrial level. Like you said, higher risk along with added shipping cost keeps them out of the mainstream. Also the need for lightweight UPS systems for servers is pretty low. One bad thing if a lead battery is involved in a fire is the spreading of lead contamination, that's not cheap to clean up, but it's low probability.

Thank you!

I'm not a battery expert, but I have quite a bit of practical experience repairing UPSs. Battery failures are a very common problem, but battery fires are not. The most common failure mode for a lead acid battery is it just stops working. Sometimes some acid leaks out. Compared to lithium based batteries, they are very safe.

If you do buy a UPS, get one with a good warranty from a reputable supplier. In my (biased) experience, it's quite common for the internal electronics to fail. The repairs are usually pretty straight forward, but it's easier under warranty :)

What suppliers can you recommend?