Chameitz that was owned by a Jew over Pesach cannot be eaten after Pesach. This poses a problem when stores or their distributors are owned by Jews and do not sell their Chameitz. In such stores one must wait until we can safely assume that food from Pesach is no longer on the shelf.
The Star-K has a list for local stores in Baltimore where chameitz items may be purchased immediately after Pesach and where it can be purchased after we can assume it is no longer on the shelf. Here is a link to the list: https://www.star-k.org/articles/wp-content/uploads/2018/03/StoresAfterPesach_2018.pdf