I dug through links I had from about six months ago, but all I could find was a company selling lenses for Holmes-style viewers:
http://www.berezin.com/3d/lens.htm

They state it is about 200mm in focal length, which as best I can figure is somewhere around 5 dioptre. However, never trust me with numbers - math and I are not on speaking terms!

Dr T's ebay store mostly has plastic-lens MF viewers.