What is the strategy to identify natural experiment as a PhD student? As social scientists become more concerned with causal inference, a good thesis these days needs some sort of quasi-experimental strategy. For example, an economist wants to know the effect of military service on income but faces the problem that people who enlist are different from those who don't. So he uses the random Vietnam War lottery as a quasi-experiment with a valid control (who won the lottery and did not serve) and treatment (who lost and did serve). (More examples of these natural experiments).

As a PhD student sitting in my cubicle, I'm lost on how to identify these opportunities. Reading published work does not really help since these experimental opportunities are quite idiosyncratic. I'm not averse to going out there to find my own opportunity, but unsure about how to do this effectively. Do I read history book? Or talk to policy makers?

Since experimental opportunities are not available to all (or most) topics, I'm already paralyzed at the stage of choosing a topic (and thus can't start reading history book / talking to policy makers). 

The standard advice I've got is not to pick question based on method. However, I find this quite a double standard given the concurrent push for quasi-experimental design.
Don't sit in your cubicle!

Go to the library (a physical one assuming you have one still). Read the newspapers (physical ones are easier to browse but you can do this on line).  Look at the controversies, the scandals, that are emerging daily on the news: political lies, child abuse, discriminative policies, public funding, privatization, quantitative easing, bank bail outs. Look at states with different parties in control. Get hold of the latest and previous census data. Get hold of the Enron emails. Get access to the data that companies are building up through loyalty cards and credit cards.  Talk to you med school or a disability charity about mining their data.  Have a look at the data mining literature - talk to the people at your university who do this and what contacts they have...