You can use a regular CR2032 battery as a replacement if you add a diode to block the recharging circuit. Full info on this hack is on Aminet and elsewhere, and DJBase here at Amiga.org has made some PCBs with CR2032s and diodes that can be dropped in directly. The battery isn't critical to operation, though.
Make sure any trapdoor RAM you buy has 1MB onboard. Most of the trapdoor RAM cards out there are designed for the 500-non-plus and contain 512K, but are sometimes confusingly referred to as 1MB expansions since they bring the total RAM of a regular 500 to 1MB (500+ has 1MB out of the box). Vesalia has full 1MB cards.
