How can I get help finding a specific textbook to purchase in the iBook store?

Have you used the search function with the exact title of the book? What is the exact title?

Are you sure that the book is carried by Apple's store?

Is there some reason you cannot provide the exact title here as was requested?

If the book is not in the Apple store, you will have to get it from another one, such as Amazon, Nook, Kobo, Google, Sony. Each of them has apps to read the books you get from them.