You might like
Freeciv. I haven't tried it yet, but I hear GNU/Linux users raving about it sometimes, and there are Windows installs. It looks to be somewhere between
Civilization II and
Civilization III in terms of complexity.
I'm not sure if you're saying you can't find
Civilization III, but if you do find it, check out
Civilization Fanatics. That's where all the addicts were going back when the game was worryingly popular, and the strategy guides they wrote are all still up. The attention they pay to detail there is kind of scary.
Something else you might like is
OpenTTD. It's
Transport Tycoon Deluxe rewritten for several platforms including Windows. You need a few graphics files from the original game that they couldn't distribute, but torrents aren't too difficult to find.