I went to Radio Shack to get mine and all they had was some short ones and larger in dia than I wanted.
Here is where I got the ones I use on the engines I make, when I reorder I will get the 1/8"x1/4 and not the 3/32"X1/4 although they work fine it takes a little more figureing to allow for the Dia of the magnet when placing the hole in the correct place.
I went with the Neodymium Magents due to the strength prolly an over kill, a good rare earth magnet will work fine. Just remember to pass it under the hall sensor to see which way it needs to be installed, one way it will spark and one way it won't.

http://www.kjmagnetics.com/proddetai...prod=D36%2DN52