I get it from a local feed store in 25 pound bags. A lot of my adoptive homes buy it from me, I ship out 11 pound boxes pretty regularly, as it's not super easy to find.
If you'd use a 25 pound bag, you can go to mazuri.com and use the "dealer locator" to find some place close to you that carries it. But before you go, call the place and ask if they carry mazuri, because a lot of places can get it, but not all keep it in stock.
If you don't need that much food at once, I'd suggest buying it from someone. I sell it, and I'm sure other members do as well (maybe even someone close to you, so you could just drive out to their place), check the sticky "Grand master chin supplier list" on the Chinchilla Supplies for Sale section. Most chins eat about 2 pounds a month, and I can fit 11 pounds in a med. flat rate box, that lasts most of my customers ~6 months.
If you want a food you can get from a pet store, most pet stores do carry Oxbow chinchilla... it's in a red bag...
http://www.petsmart.com/product/index.jsp?productId=3135798&f=PAD/psNotAvailInUS/No - usually about 5 pounds for $18, which I think is expensive, but maybe for one chin it's not bad.