As much as I like Alton Brown - IMHO he really dropped the ball on this one because potassium nitrate comes in powder, small crystals, and large crystal forms. Without knowing which form he was using - using a volume measure could be dangerous.
Ironically - his episode of Corn the Beef
is going to be on again this evening (23 Mar 2007) ... but here is his recipe for corning beef
I have to second Uncle Bob's suggestion to consider using Morton's Tender Quick
... as does the National Center for Home Food Preservation
on their page about Curing and Smoking Sausages
- look under the section on Nitrates and Nitrites
which says in part (but, I suggest you read the entire section):
... Extreme Cautions must be exercised in adding nitrate or nitrite to meat, since too much of either of these ingredients can be toxic to humans. ...
... it is strongly recommended that a commercial premixed cure be used when nitrate or nitrite is called for in the recipe. The premixes have been diluted with salt so that the small quantities which must be added can more easily be weighed. This reduces the possibility of serious error in handling pure nitrate or nitrite. Several premixes are available. Many local grocery stores stock Morton® Tender Quick® Product and other brands of premix cure. Use this premix as the salt in the recipe and it will supply the needed amount of nitrite simply and safely.
Oh - and a quick survey of drug stores in my area - they don't carry saltpeter anymore ... they have to have a special permit these days thanks to the "teeny" bombers and such. However, you can find it online from sausage making equipment suppliers, and the pharmacist at my son's store said you might also be able to find it at a feed store.
One other resource you might check is your local butcher ... if they make their own sausages. They might be able to sell you some, tell you how much to use, or point you in the direction of a resource.