This set of water-soluble colored pencils and water brushes is all you need for sketching people in the wild.
You can order Caran d'Ache Supracolor 11 pencils individually at Dick Blick. The colors shown here are:
037 Brown Ochre
045 Vandycke Brown
063 English Red
155 Blue Jeans
The four waterbrushes are medium or large size rounds, made by Niji, which you can get from Amazon
They're filled with 1. Water, 2. Higgins Eternal Ink, 3. Higgins Sepia Ink , and 4. Sheaffer Skrip blue ink
These inks are non-waterproof, which allows them to be dissolved after they are dry, and they protect the life and flow of your waterbrush.
It also helps to have a kneaded eraser and a pencil sharpener that will catch the shavings. The sketchbook should be something that can take water media, such as a Moleskine watercolor journal