Books can be classified into many different catogaries. From the more general ones like fiction and non-fiction to finer ones like best sellers, textbooks, children's books etc. It would be best to know which catogary you are buying from as bookstores usually list their books into different catogaries.
All these books can be purchased at online book stores. Usually, they will have all these different books of different catogaries available. As competition is getting more intense, they will have to be able to serve you, their customers, well, so you can be assured of the quality service you will be getting.
Do not worry too much as to whether the books you want to but will be available online. Chances are they will, even if what you are looking for is an 'antique', very old used books. Focus on the price you are able to get your book for. There is no lack of online book stores to buy from. Thus, take your time to go through a few of them to compare prices!