It's no longer supported on PPC? Did that change with 1.3.1?
And to answer your question. It's pretty hard to find the files for older versions of minecraft. At least from my own digging. I think MCnostalgia might be your best bet. Or you could trying checking some torrents or file sharing sites. Not 100% sure about the legality of the latter. I'd imagine you'd be fine as long as you actually paid for the game. I'd do your own research on that.
The launcher is Intel-only as of the latest update. The JAR should still be fully compatable though (I say 'should' because when I ran it, it was taking forever to generate the level, which was to be expected really). I guess you know what you're doing - but in case you don't you just Ctrl+Click on the Minecraft.app > Show Package Contents and then browse through until you find the .jar.