There are a bunch of "forgotten" tire sizes, and that's one of them. Some gained favor in certain markets due to certain factors and regions, others were abandoned when availability of other sizes outweighed the specific advantages of the specific size.
Read up on the history and sizing schemes if you want to go dizzy: http://sheldonbrown.com/tire-sizing.html
As far as your situation, I wouldn't stock up unless the bike you're talking about is going to be ridden a lot. Sounds like, historically, it hasn't been, but its your call. A fresh set of tires could last many years on a barely-ridden cruiser. Maybe grab a few extra tubes though.
I have a vintage kids road bike (drop bars, gears, and all) that I offered to rebuild for a co-worker. The tires for this simply do not exist anymore unless I can find new-old-stock items on eBay.