I'm afraid you won't get it here. GOG only offers the MP3 version (44100Hz 256 kb/s CBR, Joint Stereo). Some of the tracks are indeed clipped. Mostly during the entry of percussion instruments after a calmer part. A lot of these can be forgiven, because it's only short "whooms" that are clipped and not whole sequences. Still, it's a flaw and there are also little pops, but I have to admit, that even with my high quality earphones they weren't much of a disturbance. It may be different, when you hear it loud on a hi-fi system.
As of now, I know of no source that would sell the soundtrack lossless (download or CD).

Most of the time there are no lossless soundtracks provided by GOG, but it's already on the communities wishlist. Won't hurt to give it your vote.
http://www.gog.com/wishlist/site/add_flac_or_other_lossless_formats_as_a_soundtrack_option

EDIT: I dug a bit deeper and searched on the bonus DVD of The Witcher Platinum Edition.
Actually there's also no lossless soundtrack on board, but the MP3s are encoded in another format (44100Hz ~196kb/s VBR, Joint Stereo; done with LAME3.97b). In my opinion, while still flawed, they sound a bit better than the download version. I checked the files and they yield less clipping. For the downloadable soundtrack, somebody fiddled with some sort of level gain during encoding, hence it's a bit louder but contains more clipping. I hope they didn't just re-encode the old version to get rid of the LAME codec tag and VBR while raising quality to 256 kb/s to make it look better :-/ We'll probably never know.