How do I get my firmware version prior to running jailbreak on a new 3GS?
Recent versions of redsn0w auto-detect the version of iOS running on the connected device, so you can usually just jailbreak it without having to manually figure out the iOS version first. You can always get the latest version of redsn0w from the Dev Team Blog: http://blog.iphone-dev.org/tagged/redsn0w

Jailbreaking with redsn0w also "hacktivates" (unofficially activates) unactivated iPhones (although not the iPhone 4S yet), which is convenient for these kinds of situations. Some things don't work on hacktivated iPhones (push notifications, iMessage, FaceTime), and the battery may drain more than usual, but it's fine for a temporary solution when no official SIM cards are available.