Did you open the door where the RAM lives, then look to the side?
There also is a PMU re-set button on the logic board in there
to reset the power management unit manually, next to RAM
and near the 3.6V 1/2AA Lithium battery.
On the slot-load iMac 400DV G3 you don't need to fully open it.
The price cited for this battery in Canada is outrageous, and I'd
looked into some computer web site there to see $19.99 for one.
There are several sources for these, so someone must be able
to sell one at a time for less than these incredible numbers.
When I was repairing dozens of computers to donate, I was able
to buy dozens of these batteries at 50¢ each, with minimal post
in padded envelope. At one interval, they were even less. The
most I paid (by mail) was $1. per battery, shipped first class.
You may be able to find a source for a vintage service based
manual, but you don't need one to replace a PRAM battery.
I took mine all apart, upgraded HDD, RAM, & other stuff with-
out a manual. Found later it needed an power circuit board, too.
Later when all fixed up, I donated it when I bought an iMac G4.
All total, I repaired over 220 Macs and donated almost all.
Could be if you search for images of iMac G3 slot load battery
replacement, a few of them may actually show the placement
of the battery next to the RAM chips, (via RAM upgrade door)
and it should be about that easy. Just so long as the battery is
replaced in the same polarity as the old one.
Good luck & happy computing! 🙂