What is the easiest way to run SW that requires Classis Mac OS 9?
I agree with @TomShen - SheepSaver is a good option. 

Just keep in mind that it only works with Mac OS 9 up to 9.0.4 (so don't fall into the trap of trying to use Mac OS 9.2 - the most popular version of that release). 

Also, if you're wanting to find a secondhand Mac that can run Mac OS 9, you also have the option of Macs that could run what was called the Classic Environment. This was a compatibility layer for running older Mac applications and was included with PowerPC versions of Mac OS X up to and including Mac OS X 10.4 Tiger. In other words, if you want a dedicated Mac you're not necessarily limited to much older ones that could run Mac OS 9, but also have the option using the last models of the following series:


Power Mac G5
Mac mini G4
iMac G5
eMac
iBook
PowerBook G4 


NOTE: These systems cannot run MacOS 9/Classic applications if running MacOS X Leopard or above!