i see three options
1. I-81/I-75 to Cleveland TN then across US 64 to Murphy (approach from west)
2. I-81 to Abingdon VA then across to WJ, if you have lots of time and don't mind the twisty mountain passes
3. I-81 to I-26, past Asheville, the across US 64 to Murphy (approach from east)
Option 1 takes you through more All-American territory in east TN. You also get to see the Johnston Coke plant in Cleveland. See my post from 10-8-06 re: the tri-city area of TN. You can find a lot of longnecks in Abingdon VA. If you decide to go as far as Chattanooga, that is Coke United territory and they occasionally have odd stuff.
Option 2, if you want to go to WJ:
You can get all the Tri-Cities Bev. stuff (Nugrape, Nesbitts, Dr Enuff, TC's) at DPWJ. If you decide to visit WJ be prepared for long drives on very crooked roads through the mountains. You can take US 58 east from I-81 @ Abingdon VA and and then south on US 16 from Mouth of Wilson to West Jefferson. Check weather first because the mountain passes especially White Top in VA can get slick in November.
I would not recommend trying to cross the mountains on either US 421 or US 321 from TN into NC at this time of year unless you are taking it in the daytime. 20+ miles of hairpin turns, on unfamiliar roads, in darkness wouldn't be great fun. Oh, and did I mention the critters in the road?
From WJ to Murphy - well that's gonna be a challenge time-wise any way you look at it. Either more roads through the mountains or zig zag down to I-40, west to Asheville then south to Murphy.
Option 3 takes you through Bristol, Johnson City and Asheville. An easier ride but you'll need to look for the sodas, they won't just jump out at you like here in WJ.
If you want to buy in volume you need to go to WJ. If you want to find small quantities of different stuff, any of those options would work.
Hope this helps.
90 years of Cheer...1917-2007