I remember an article in Our State Magazine. When I found it online, the author said that the vendors are elusive. The pop up over-night and are gone a week later. She did mention beside the Smithfield's in FV along I-40, and along I-70.
However, she also provided a recipe for DIY: "Empty a bag of raw peanuts into a slow cooker, add water to the top, stir in a cup of salt, and cook on high for 24 hours."
Here's the rest of the article:
On a Quest for Boiled Peanuts