You can buy it off of GOG.com (run by the company that made The Witcher). IIRC, most games on their site are around $5.99, but they are always DRM-free, and re-jiggered to work out of the box with Vista or XP (no DOSBox or workarounds needed). Even with the re-jiggering, most mods work fine for games downloaded from them.
Edit: Looking at their site, it's the Atomic edition, which is supposedly the most complete form of the game. Also, I was right about mods, a GOG employee even posted a list of good mods in the forum.
--Too Dumb to Quit--