Rather than purchase, I would recommend you rent your textbooks. That is possible here where I am in the Dallas Texas area, and for the past 3 years I did in fact rent my textbooks for about 1/3 of the cost of used textbooks, which is what I usually bought. It may be possible to rent in your area too.
I used to go to my local bookstore that handles college textbooks early, several days before the beginning of classes when the selection of used books was ample. Invariably by sorting through the selection available, I could easily find books that were so new, they even smelled new, because their previous owner never even opened them! Technically they were used, but they looked, and smelled new. I paid less than half the new price. Even so, renting is even cheaper and I highly recommend it.
If you insist on purchasing new textbooks online, just Google "textbooks" and you should have no trouble bringing up dozens of online sellers. Make sure you get the textbooks, correct edition, etc., that your professor is using.