Wood-cased drawing pencils are produced in a range of grades (of graphite/clay mix). These are 12B (softest-least clay) through F and HB to 10H (hardest-most clay). As you would expect, the softest grades deposit the most graphite on the paper, resulting in the darkest lines. Also, the softer grades have a thicker graphite core to reduce the potential for breakage and so they can produce broader lines. The grades available vary from each manufacturer.
It is always advisable to buy good quality art equipment. Cheaper pencils are often composed of inferior materials. Wood stems that split, or are not bored accurately to take the graphite, can cause problems when you try to sharpen them. The graphite/clay mix needs to be ground finely to give a smooth, consistent delivery of pigment. Slivers of un-ground clay in the pencil can scratch the surface of the paper and also result in loss of pencil-life as you have to constantly re-sharpen them to remove offending sections.
The graphite pencils that I have included below are all high quality artist grade pencils which regularly rate highly in pencil reviews and can be found for sale from most art suppliers.