I had a PowerBook 100 a few years back and I searched long and hard and finally found a BRAND NEW SEALED battery never used, on eBay, Apple-branded PowerBook 100 Lead Acid battery. It arrived, I installed it, and found out that it was dead.
So even if you find a new one, chances are they will be dead by now since they are so old (20 years old) and nobody makes them anymore, at all. You will probably not be able to find one, there is no demand for them. They still do make the batteries for the 140/145/160/165/170/180 series machines which are NEW and can be easily found on eBay -- but these are NiCd batteries, not Lead Acid. The Macintosh Portable batteries can be easily rebuilt with new cells, however, and function as new batteries. The problem with the PB100 batteries is that they are sealed and you cannot replace the cells.