My favorite banh mi baguette is made by Catania Bakery in DC (which I think is wholesale only) and is available very inexpensively at Litteri (in DC), Huong Binh (at Eden Center) and a huge bin delivered fresh daily at Oriental Supermarket 5001 Columbia Pike Arlington.
I'm sure there are many more, but those are the ones I have seen personally.
If you're near Eden center, you can buy appropriate bread at several of the bakeries there. Of course, it is tempting just to go ahead and buy the sandwiches themselves.
Or just use a good quality baguette. Last time, I think I got a couple of extra thin, very crusty, baguettes from Cheesetique to make banh mi for a picnic.