Assuming there isn't a backstamp (name of maker) on the underside of the foot, and that no one recognizes the pattern, then id can be very difficult - there are literally thousands of patterns of cut drinking glasses, and made in many countries.
Whilst you're waiting patiently, you might try a States based internet replacements retail outlet by the name of Replacementsdotcom - just put a dot instead of the word - of course you might have already tried that and come up with nothing. In which case it might be a lot easier to buy your parents another full set of twelve glasses in a pattern you consider they might appreciate. Just as a matter of interest, though I can't imagine for a moment it will help, approximately how long ago might the set have been purchased? I mention this as some of the once best known names in cut glass production, are now no longer - and for some years before their demise had their production sourced from outside the U.K. - this give rise to disappointment when replacements are purchased and found to differ in size and possibly pattern too. So a little caution needed even assuming you do find what is listed as the identical pattern.
Best of luck.
