ebay has one currently listed for $75. I think I paid about $25 last summer for mine and the allen wrench/hex key part was too large. But it isn't a big problem to find a separate 6mm hex key/allen wrench to carry along. I've heard some folks modify a flat 10mm wrench that cyclists use. But I haven't tried that.

eeuroparts.com has one for under $25.
or pelicanparts.com for under $25

Just google "convertible top tool mercedes benz" and you'll find something that will likely work....