The quickest, and easiest way to get hold of a selection of pastels is to buy a ready-made set. All of the major artists' quality pastel manufacturers do sets (see Which are the Best Pastel Brands). These range in size from ones as small as six sticks, to large wooded boxes covering their full range.
If you just want to try out pastels and get a feel for them, then get as small a set as possible. Or, better still, consider buying several sticks, each from a different manufacturer, so that you can experience the range of pastel softness/hardness available.
If you want to try some serious pastel painting, you'll need to get a set of between 30 and 40 pastels. If you already know that you intend to do mainly portraits or landscapes you can further refine this choice by buying a targeted pastel selection (starting with 10 midtone colors.
