I recommend purchasing directly from the developers that ported the game to current OSes:

http://www.wyrmkeep.com/ite/

Already bought it from them years ago, the CD version either glitched on me or I took actions it didn't expect and skipped a big chunk of the game XD

Personally I recommend GoG cause they keep the games well patched so may be more futureproofed [certainly more so than a physical CD.]