Where can I buy......????

All you can do is search and contact different pepsi bottlers location.  Each one has their own franchise areas, and each one may handle different products in that area.  
As an example, the Walmart on the east side of Appleton is in the Oshkosh Pepsi franchise area,  The Oshkosh Pepsi does not offer Caffeine diet Mt Dew. However the Walmart on the east side of Appleton is in the Sheboygan Pepsi franchise area, and they do offer the Caffeine diet Mt Dew.
that is why you can find it in the Sheboygan area.  
Because it is not a high volume sales item is why most pepsi bottlers do not offer it. Another reason is that it is a short self life product. Even in areas that do offer it, you would have the best luck with larger stores such as walmart, festival, woodmans, etc.  
GOOD LUCK