In my current game, I am Greece (Scientific and Commercial), I have spent the whole game concentrating on money and science and in some cases I have been able to hit 4 turn research. I know I have been first to reach several techs first, but not all. Still, I have yet to get a science Great Leader. I am up to refining on an emporer game.
I would say if you want more science leaders; first, you need a scientific civ. Then you need 'more chances' to be first. That means, colossus, libraries early, then currency, then republic early. Then, if you can get out ahead, you need to go for techs that are cheapest and fastest while trying to quess what the ai will go for and possibly avoiding what you think they will go for.
Beyond that, you may need to beat the ai out of republic and into monarchy/communism by instigating wars. You may need to pillage your rival's river vallies in a major way in order to reduce their science out put. And keep the ai fighting itself while you build population/science and money.