The tool for the old Shimano freewheels is available from "The Bikesmiths" for $8.50 + shipping. A couple of ebay sellers have them, one for $8.50 + $1.42 shipping which works out to be cheaper. I recently bought one from the Bikesmiths and it works.

From the looks of your freewheel, the old, small diameter Shimano/Kengine remover is probably the right one. You can see that the bore on the freewheel is pretty tiny. You will need to remove the nut to get the remover on there.

Unless you're like me and have an unfulfilled desire to own at least one of every version freewheel remover ever made, or, unless you just love that freewheel, the better, cheaper, and quicker solution is to take bluehills' advice and take it apart and wrench/vise it off.