You can get at your local Asian store. Sometimes they have different label "Tapioca Starch" or "Tapioca Flour". Asian stores have many brand brands such as Asian Best Brand, Erawan, etc. I'm pretty sure you can get it on Amazon or online stores if you don't live near Asian stores.

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.

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 :)

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.

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.