The crystal bag was in act 1, in the room where picking up a key triggered the appearence on a teleporter pyramid, just before getting to the citadel.
You should be able to get the crystal bag by downloading and swapping inventory files, as described in the topic below.
My game was started in an earlier version and saved under 1.44 when I checked this. I tested in a save in act 3, and was able to give both characters a crystal bag (though with some irregularities in how game handles resistance boosts, since it is only expecting a single bag). It also worked fine when I loaded a previous save version, saved after passing the crystal bag, and did the file swapping to get it back.
crystal bag (10KB) - inventory files
Lost Items - guide and directory of inventory files (written for DD, works for BD)
