For a real book lover, there is no substitute for having a solid, well-bound and long-lasting volume in your hand. So while Perseus' version of Lewis & Short is welcome, what is really indispensable is an actual print copy. (Mine came at the miniscule price of IR£20 in the Blackrock Market in 1992!)
However, it's not always possible to get every book you want, or even to afford the ones you want most. Here's where the Internet Archive comes into play, more specifically their Text Archive. They will give you flip books i.e. readable facsimile images of the book concerned which you can look at online. Or you can get full colour PDFs of many texts.
The two books I've wanted for some time but could never find were the Stowe Missal in the Bradshaw Society edition and the original critical edition of A. M. S. Boethius' Opuscula Sacra by Rudolf Peiper. (Leipzig: Teubner Verlag, 1871)
In both cases, I still don't have them and may not for many years. What I do have are colour PDFs so that I can read the texts I need to, and get the research done! It's not perfect but it's a whole lot better than no text at all. Now if only someone would put Lewis & Short up there; that way I could carry a copy around in my laptop!