Don't confuse the sesame paste used in Chinese cooking with Middle Eastern tahini, which is also made from toasted sesame seeds, as BrooksNYC says in a recent Chowhound thread. The flavor is quite different.
Chinese sesame paste can be found in Chinese or general Asian supermarkets, in both black and white varieties, cpa314 says. Black sesame paste is commonly used in desserts, while the white variety often shows up in savory cold noodle dishes, cpa314 says.
If you can't find proper toasted sesame paste, JungMann gets great results from mixing tahini with a little toasted sesame oil; BigSal suggests substituting peanut butter. "I've read elsewhere that authentic takeout flavor calls for Jif peanut butter!" BrooksNYC says.
Discuss: Asian Sesame Paste