The 'Plum sauce' for Moo Shu Pork is actually not a plum sauce at all.
It is called Hoisin Sauce, an is made from a combination of soy beans,
garlic, chili, and other spices.
My recommendation is that you buy a container of it from a ethnic food
store. If none are nearby, I know it is accessible at almost any
grocery store. I bought a large gallon container of it for $6, because
the small bottles of it cost about $2 fo 12 ounces...
Hope it helps...
Dave Hrencecin