Just need wheel assembly

We are looking for 2 wheel assemblies for an older set of mirrored closet doors. One door has two complete sets with wheels and works well but the other door is missing two of the slide on wheels. We have the part that is screwed into the closet door, just need 2 of the wheel parts that slide on (can be seen clearly in 3rd picture). There are no markings on the wheel assemblies. Have done searches online but cannot find the part. If the part is no longer made and can't be found, is there something else we can replace it with?