They are quite hard to find. I've seen some on eBay, but like most everything else there they are overpriced. There are a few companies that still make them on a specialty basis, but they have for the most part,been replaced by the calculator. I'd keep it, specially if it has any sentimental value to it. It would be quite a curiosity for your sons I am sure.
It's quite a curiosity for me, too. :-)
Maybe I'm getting old, but I remember when calculators were a new thing and very expensive. Now I can pick one up at the dollar store. Who would've thought that one day calculators would be so cheap, and rulers would be hard to find.
I'm always shopping the educational stores and catalogs. If I come across a real slide ruler that's inexpensive, I'll let you know where to find it.