It seems like it was about a year ago that we had a similar discussion. Here is what I remember that I learned from that ...
1) You're going to be hard pressed to find saltpeter in a pharmacy these days. Since it can be used to make bombs there are a lot of new government regulations on its sale ... and every pharmacy I contacted simply quit selling it - and will not order it for you.
2) The difference between Food Grade and Pharmaceutical Grade depends on the regulating agency. Food Grade is reguladed by the FDA (Food and Drug Administration), Pharmaceutical (drug) Grade is regulated by the USP (United States Pharmacopeia). Like with mineral oil - both are equally consumable.
3) The potassium nitrate you get from a gardening center is NOT food or drug grade and should not be used in the preparation or preservation of food. Same goes for the Lab Grade you can find on the internet.
4) The site that Auntdot posted, along with some other sausage supply sites, used to carry potassium nitrate about a year ago - but a search on their sites now comes up empty.
BubbaGourmet - you may want to read this article at Meat Science & Meat Sense
(read the section under Nitrates and Nitrites). Since "corned" beef is salt cured beef (the "corned" goes back to the English term of "corn" used to refer to "grains" of things ... on this case it's grains of salt) you might want to reconsider Uncle Bob's suggestion of using Morton Tender Quick
which is very easy to find in most grocery stores. You might find the Homemade Corned Beef
information at this site to be of some help.