Stephen,
 
I have a regular incandescent bulb in my shell cabinet as I like the
warmth of it as compared to a florescent.  Almost any lighting will do
as it is the usually thought that UV is what bleaches out shells and
commercial lights have very little to no UV (unless you pay extra and
buy a special full spectrum bulb).  So go with any cheap light and enjoy
the results.  The shells will fade more from the normal daylight
exposure than from your evening lighting.