I found one at a thrift store, and am selling it. It may or may not work – as the key is on the box, but who knows if Adobe is having the software phone home.
According to Adobe’s site, the requirements are:
PowerPC G4 or G5 or Intel processor
Mac OS X v10.4-10.5
768 MB of RAM (1 GB recommended)
1 GB of available hard-disk space
1024 x 768 monitor resolution
So you need to be using OS X 10.4 or 10.5. That’s a pretty narrow band right there.
OS X 10.5 was called Leopard. So if you have an older 2007 or so era Mac, you can use this old software.
I think you can find older Macs for sale for not that much money. I see a Mac Mini on Craigslist for $120, which seems steep to me. I saw a similar Mini, but with a G5 CPU, for $30 at the thrift.
The other option is to use an emulator, and load in the old OS there.