I have a set made by Sakura Color Products called Carre Pastels.
I bought this as a set of 12 and it is called a basic earthtone ensemble. It includes various shades of browns, black, & greens. In the future I will probably only buy single sticks because I find the browns are used much more.
Be sure to get the pastel chalks ..... not the oil based pastels. They are just a glorified crayon and don't work well for weathering purposes.
Best place to look are Arts & Crafts stores like Michael's.
Happy Modelling and God Bless
Robert