Flywheel retention tool

[01_SuperSlow]

I,m ready to change my clutch and, as far as I know, I need a flywheel retention/locking tool so I can unbolt and remove the flywheel.
I went looking for it to pepboys, kragen, autozone, sears, snapon.com, and napa. They don't have it in any of these places.
Where did you do-it-yourself guys get it? I'm out of ideas.
It is necessary right?

Thanks

[jmd]

Snap-On
Stock Number A144A
Name Flywheel Turner
Price** $69.25

Frankly I make do with a screwdriver wedged against the block or some engines I can turn the flexplate by hand. But it is a pretty handy tool if you're going to be doing several transmissions.