Boiled Green Peanuts
You can buy raw peanuts by the bag at How's Markets (they also have them in a bin to scoop any amount you want). I'm a Southern gal, been here for 14 years, and have never found them in SoCal. Albertson's used to also carry the raw peanuts by the bag in their produce section, but they've recently changed vendors and they told me they no longer carry them. You can also order the raw peanuts online (just Google).
In So Cal, the season is October through December. I've heard in Georgia that the season is April or May, but not here.
As far as a roadside vendor that sell them, that's unlikely to happen. I've had them as a beer snack at some LA restaurant some years ago, but I can't remember where.
To boil your own, check your local farmer's market for the best green peanuts. The Hmong farmers at the UC Irvine has them in season.