The first version of Mac OS X appeared in 2001 and was numbered version 10.0 it had the name "Cheetah". Subsequent releases increased the point number and had a big cat name.
They were:10.1...
To find the version of the Macintosh's current software, pull down the Apple menu (upper left on every screen), and choose the first option: "About This Mac".
This is one place, although the prices are rather high, in my opinion
http://hardcoremac.stores.yahoo.net/
Other resellers still carry them from time to time.
If your machine is capable,...