CellsByTag[notebook_, tag_] := Module[{cells},
  cells[nb_NotebookObject] := cells[nb, _];
  cells[nb_NotebookObject, pat_] :=
   Flatten@cells[First@NotebookGet[nb], pat];
  cells[cellList_List, pat_] := cells[#, pat] & /@ cellList;
  cells[Cell[CellGroupData[group_List, ___]], pat_] :=
   cells[#, pat] & /@ group;
  cells[c_Cell, pat_] := If[MatchQ[c, pat], c, {}];
  cells[__] := {};
  cells[notebook, _?(MemberQ[Flatten[{CellTags} /. Options[#]],
       tag] &)]
  ]