All the time!!!! David Sandler has talked about the three foot rule. Basically, anyone that is within three feet of you is a prospect. He suggested that if they are that close, you should talk to them. I have always felt that no matter where I am, there might be someone near me that needs our services, or they know someone who does. But how do you get to the point that you are having that conversation?
For me it just starts with an ice breaker and some small talk. Usually after I have established some bonding and rapport, I usually ask something like "So when you're not here shopping (or whatever it might be), what do you do?" People love to talk about themselves and we are off an running.
So no matter where you are, keep your eyes and ears open for opportunities to make connections.