It took some tooth pulling, but I finally identified the battery used in the POCSAG pagers you can get on eBay from China. They don’t come with the battery because of shipping hassles. It’s an 802040 LiPo cell, 3.7v 600-700mAh. There’s plenty of them on Amazon.
Update: Wait til the pager comes in. The seller may be freaking clueless, and it just takes regular old AAA batteries.