If you have a particular title in mind, go to the publisher's website. Sometimes prices will be listed there for a particular quantity of books.
Go to your local bookstore. If have a book club based on a different reading each month, ask if they will be willing to sponsor it by donating books. Sometimes you can walk away with a free place to meet and free books each month! Even if they are not going to give you books, you can ask for a discount and let them know how many of each book you will be purchasing next month. The bigger the quantity, the better the discount.
Check with your library. Like bookstores, libraries are able to purchase from distributors and wholesalers at deep discounts. If you use the sponsorship scenario, this will allow you to have a free place to stay, advertisement for your book club and discounted or free books. You can offer to donate copies back to the library that participants do not want to keep.
Look online. There are many retail outlets online, especially on Amazon, that will allow you discounts on certain quantities of items.
Compare your results from all available outlets and choose the option that works best for your situation.