Hello,
Please find some suppliers that carry books-
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To find specific products on their sites, I recommend using the search function or browse through their categories.
Make sure you contact them directly in order to get their true wholesale pricing which is always better than you will see on their site. When doing this, provide them with the quantity you are ideally looking for, where you want them shipped to, and how often you plan on purchasing - this helps gets the best possible price.
Let them know you're selling on Amazon as well so they will have an idea about the conditions you're dealing with Amazon. Know that different suppliers have different terms so it's best that you communicate your concerns directly with each supplier you deal with.