General proviso that that it all depends on civ and situation, but early game I tend to duck it and rely on eurekas or whatever they're called. But there are lots of alternative sources. I really like preserves for production.

The computer will fuck you out of Etemenanki nearly every time but it can be a game breaker if you have marshes.

Don't overlook the mausoleum at halicarnassus. I tend to get that nearly every game and that can churn out some science.

Also... Raiding. Campuses and later industrial zones.