When older out-of-copyright books are assigned, you may be able to find free full text online. An internet search for the title in quotes with the words full text may find it [example Google Search: "Ragged Dick" "full text"]
You may find multiple copies of a work on these sites. A good strategy might be to provide links to more than one source of the book. This lets students choose which version they like best--some are simple text and some have a "turn-the-page" feel, etc.
Below are links to sites that offer full text of classic books.
It's a good idea to also stock print copies at the book store for students who prefer print. Some high quality editions of classics might be found with university presses (see University Presses & Scholarly Publishers page). The following publishers produce inexpensive print versions of classics: