Shells:

By far the most common unfinished shells out there are the Keller Maple Shells. A drum shell normally will simply be a maple plywood tube with no holes and the ends simply cut off square. They come in a variety of thicknesses and standard diameters that will match commercial drumheads.

Usually if you want a less deep drum, the suppliers will saw the shell shorter for free (or you can saw it your self and keep the extra piece for another project).

For extra money many suppliers will cut bearing edges on the shell and drill the shell for lugs (especially if you are using lugs that they sell). Going this route reduces the "drum building" project to simply a wood finishing job.  

Shortening a shell:

If you want to cut down an existing shell or a new one, use a table saw with a fine-toothed plywood blade. Use the fence to set the new shell depth. Carefully rotate the shell to cut off the excess.  Be sure that you do not leave a "step" in the cut or you will have trouble when you try to cut a bearing edge. If there is a tiny "step" sand it out.