pj, all I can suggest at this point is to try other pharmacy chains until you find one that carries a different brand or type of combination set. Or just forget the combo entirely and buy a fountain syringe that is used only for douche or enema (never both!).
Again, Faultless or Davol brands shouldn't have this problem, but they are often in the $15 or so price range. Try CVS and Eckerds if they are in your area.
Hope you find something reliable soon. An enema when you are relaxed in a comfortable position and not worrying about leaking or anything else will be far more effective than one when you are tense and worrying.