Congress Tools, Inc sells 1/2" width triangular polishing stones in rough grits that can fit in the Sharpmaker slots if you want an alternative to the diamond stones. The Congress stones aren't precision ground so some might be a bit wider or a bit thinner. If wider you can just whittle away bits of one end until it fits into the slot. If thinner you can just wrap one end with plumber's teflon tape till it fits.
I recommend their Ruby stones because it seems like they're the fastest cutting designed for ferrous and other hard metals.
The Ruby selection is here:
http://www.congresstools.com/catalog/categories/get-subcategory/?id=24
I bought several of the Ruby 1/2" x 6" at 60 grit and a couple at 100 grit. The 60 grit cuts very quickly.
If you want to try their other stones, the main page for their stones are here:
http://www.congresstools.com/catalog/categories/get-category/id/72 .