On page one Peter Schrager suggested x-ray dup film. I second that. I have used that for making enlarged negatives for pt/pd printing and can say that it is excellent. The chemistry is simple Kodak GBX, it is a reversal film so you can enlarge as making a print. Remember to reverse the negative that you are enlarging (emulsion up in the negative carrier) and every thing is reversed so if you burn, the new negative will be "lighter" in that area and the resulting print made from the new negative will be "darker". Dodging will yield a "darker" new negative and a "lighter" area in the resulting print. The film is very slow so when enlarging the exposures will be rather long. The tonal range is great.
You will probably have to get it through a dental/medical supply or have your dentist order for you. It is not cheap but the results are worth it.