AFAIK, these batteries are no longer in production. Electronic retailers, and hobby shops don't carry them, and I doubt one can purchase a new one online.
Personally, the 4.5V batteries were a joke, especially the Rayovac ones found in the 5xxx Performas (although that one can be easily replaced by soldering three "AA" batteries). They don't last as long as the common 3.6V lithium batteries, and as far as the early Compacts are concerned, impossible to find nowadays in working condition.