Preposition

As Tolley said, either is correct. Each conveys a different meaning.

Prepositions are not placeholders. They express meaning in and of themselves. "In" expresses "the situation of something that is or appears to be enclosed or surrounded by something else." 

"The books are available in the ABC store" might also be "the books are available inside the ABC store."

"At" indicates "relative location, presence, occurence, or arrival in a particular place or position in, on, or near." At is less specific than in -- at does not tell us whether the books are inside, outside, or next to the bookstore.

In many cases, you can substitute "at" for "in" -- making the sentence less specific. However, "in" cannot substitute for "at" unless you could also use "inside."