The tool is about $17 from Performance Parts and there is a stubby Stahlwille 5mm allen (~$7) that you should purchase along with it if you don't have a set of stubby allens.

I just used these tools yesterday and wouldn't have gotten the clutch off without them. On my 1991 300CE, you use the tool from the 3 o'clock position. You are searching for one of two holes in the back of the fan clutch pulley to insert the tool.

glenmore
1991 300CE
2000 C280
1990 LS400