Where can I buy tapioca flour
Tapioca flour is found in a 'flour' or 'specialty baking' section of a health food store. I also buy it at Asian groceries. If the amount you need is small, try cornstarch or arrowroot as a substitute.
And it is sometimes called Tapioca Starch but it is exactly the same thing. Many Whole Foods carry it. Asian groceries are another good source.
I just bought some at Wegmans, took me a while but I finally found it in the gluten free section. Try that at other stores if you're not in an area blessed with Wegmans :)
Pretty sure you'll find it at Whole Foods in whatever section they have shelved the Bob's Red Mill products. I know they make one.