Shouldn't matter which iTunes platform you're working with. You can download older .ipsw's off the internet - just make sure you get a reputable one. There are quite a lot hosted out there, sorry I can't point you to one right now. I remember finding mine by searching for links in jailbreak forums - people will tell you where the gooduns are. Then you just have to drop it in the same folder as your existing .ipsws, and do whatever the winxx equivalent is of option-click on the Restore button in iTunes, which then offers you the chance to choose your .ipsw file.