Where Can I Buy A Manual Lawn Mower?

I have a gas lawn mower, but it seems like a waste to use it, when I could use a manual mower and get exercise at the same time, with the gas savings.
  But they lacked the local big-box hardware store. They looked confused when I asked, and tried to sell me a riding mower instead.
  Where can I buy a manual push mower for our times?