Anyone can purchase biology textbooks online through publishers' websites, such as Pearson Prentice Hall's. Educational publishers usually offer textbooks spanning a wide range of topics in addition to biology.
Depending on whether the book is for personal use or it is part of a bulk purchase, websites such as Prentice Hall's provide information on how to acquire funding from various sources and use it to make online purchases. If the book is a single copy for personal use, the websites typically function like any major online bookseller.
Alternatively, a simple search of websites, such as Amazon, may reveal new and used copies of biology textbooks for sale.