I've just received a catalogue through the post from a herbal co. They have a special wet and dry grinder on sale especially for people who want to put certain herbs in capsules that are only sold as cut herbs. £23 though, a bit expensive!
Timely eh!
Keep in mind though that some grinders are going to hard to clean. And I have had grinders gum up and lock up from griding certain herbs with a higher sugar content. The sugars get sticky from the heat of the grinder. Resinous herbs such as chaparral may pose the same problem. Check for reviews of the grinder to see what other people thought of it first.