Finding the formula
The note can be found inside the Nuka-Cola plant in Fallout 3. Upstairs in the office section, near the shipping section, you should find the body of Winger Mercier. When you search him, you will find this note which points you toward an optional objective of The Nuka-Cola Challenge.
The Formula for Clear Nuka-Cola is on the first floor of their D.C. Plant. That's where they did all the R&D for it. Get the formula and meet us at the Red Rocket Tricycle Factory as we planned. Our buyer is promising lots of caps, so I want to see some hustle! This could take our gang to the big time. Sudden-Death Overtime forever.