The Georgia Book Company runs a site called teachereditios4less.com, where people can find teacher editions of many textbooks and workbooks. Other booksellers often have the teacher editions in stock, and those editions can be found by searching for "[Textbook name] Teacher Edition."
Amazon.com, for example, keeps many teacher editions for people to buy or even to rent for a time. AbeBooks.com also keeps teacher editions in stock. Simply put, most booksellers that have textbooks and workbooks are likely to have teacher editions in stock, as well, so search the catalogues of those sites or stores, and the editions are probably there.