Pit beef is almost always made with signature Pit Beef Spice which is a dry rub blend of ingredients and for best results is rubbed on the meat and marinated in the fridge. This does have a somewhat salty base but when mixed with other ingredients soaks into the meat and makes it yummy.
MAKE YOUR OWN
You can also make your own custom blend and see how it goes. Start by using Big Fat Daddy's famous dry rub seasoning as a base , because the recipe was blabbed here to the NY Times (well, almost--I'm sure they didn't tell all.)
WHERE TO BUY
You can check with Big Fat Daddy's store to buy some, if you are lucky enough to get your hands on it. UPDATE for 2012: Try on Etsy here.
Otherwise you can look online for any kind of beef dry rubs. No two taste alike . The best part of dry rubbing is trying different blends to see which one you like best. The sky is the limit when using different dry rubs for your pit beef!