Sriracha
A hot, slightly sweet chili sauce with garlic and a little vinegar, smooth and thick.
Sriracha is now stocked in many supermarkets near the ketchup or in the Asian aisle, and reliably in Asian grocery stores. If you can't find it, another smooth chili-garlic sauce such as sambal oelek with a pinch of sugar will do.
Substitute:
Sambal oelek with a pinch of sugar and a little garlic comes close, though it is chunkier and more sour; another hot chili sauce also works but brings less sweetness and garlic.