Most Vietnamese restaurants will have banh hoi even if it's not on the menu. Especially any of the restaurants that sell the traditional Vietnamese tossed noodle (vermicelli) dishes. It's like an Italian restaurant that will have different types of pastas available for your selection.
Golden Deli in San Gabriel on Las Tunas has great Banh Hoi. It's usually really fresh and the best consistency. I like it better than the usualy bun. It tends to have more flavor.
Pho 79 in Alhambra and Downtown LA definitely has Banh Hoi Thit Nuong with all the fresh herbs and greens. You can also get it with the charboiled pork paste or shrimp paste wrapped over sugar cane. Hmmmm, my favorite!