Which version are you after. I assume that you're aware that you will need an unlock code specific to your GPS for it to actually work
The mapping itself, when it came on CD/DVD (now a download), was distributed free to registered owners on request. It's the unlock code you needed to pay for.
A StreetPilot III won't run the the current NT mapping, non NT mapping was phased out after City Navigator 2009. This was preceded by City Nav 2008, that by City Nav v9 (2007) and then City Nav v8 (2006) etc.