Sandy, if you're looking for the naturally black form of Mica (found under the catchall name of 'biotite') in order to get the same tonal inversion as in a tintype, you will probably have to go to a mineral dealer. It absorbs light very well, so if you can find a sheet you can always make more by cleaving it into thinner sheets still.
If you want the transparant type there are consumer applications like the stove windows mentioned by Don which make large-ish sheets easier to find. Small pieces - up to an inch square - of very high quality are also available quite cheaply from microscope suppliers.
To answer the original question: I love intaglio printing, so I would have to go with photogravure. I am still looking for colour photogravures to see if I like them as much as I think I will.