Calling FSBOs can work but isn't easy. As Brad said, you'll be one of many agents calling the sellers trying to poach their business. But many agents do this and make a good living on it. The key in my opinion is to be able to offer value. Once they have been on the market for a few weeks, call and ask how things have been going. What feedback have they gotten? Then offer to give them your feedback as a successful agent. Ask to view the home, then give them your opinion of the home, price, etc. Offer suggestions on how to stage better, have better pictures, etc. Talk to them about their home just like you would any other seller/client. Try to build rapport. You may waste time doing all of this, but you may also impress them. Maybe they will come to you if they don't sell soon after, or maybe they will refer you to someone else. Building a name for yourself by providing useful insight will go a long way.
Zillow can be hit and miss as well. Most of their leads don't turn into anything, but some do. If you can get one lead a month to turn into a transaction then the marketing paid for itself. We have found that staying in front of the lead and establishing yourself as the most knowledgeable agent in the market makes a big difference.
There is no perfect formula for generating leads. But having multiple sources is important, and buying leads is a good way to get started until you can build up your referral base. Good luck.