I have a Polar F1 (?) and could not find a battery at any of the local watch battery locations. Did find it at Batteries Plus.
The metal clip has a catch mechanism that open real easy once you see where to put the tip of your (very small) screwdriver. Be careful not to force it! I watched in horror as a clerk at on store went at it with great vigor. I told her to lighten up and let me do it.
It's an easy job. Don't really know why Polar wants to void the warranty if you do it yourself, but that's their policy. It's out of warranty, so diy for $4 sounds like the route to go.