What you want is a neodymium magnet. These suckers are CRAZY powerful.
Here's a good place to buy, I have done business with them before and they're great:
http://www.kjmagnetics.com/categories.asp
Shape doesn't matter much, but total volume does. An N38 - N42 grade magnet that is the volume of two pennies should be enough to stop most analog watches temporarily.
If you get a magnet that's TOO powerful, you risk breaking the watch. These things are stronger than you can even imagine if you've never seen them before. I bought two blocks, each the size of a sugar cube, and they are nearly impossible to pull apart using only hands.
I know a lot about magnetism (thanks physics degree), but virtually nothing about how it affects watches. Maybe buy a few of various sizes, and test them on your own watch (gradually increasing the magnet you use -- that's safe imo).