Is it permissible for a divorced woman to travel a distance of 900 km to visit relatives, buy necessities, or for a change of scenery, whether alone, with her sister, or with her sister and her husband? And does the religious prohibition against a woman traveling without a mahram apply to her?

What you mentioned about religion prohibiting a woman's travel without a mahram (definition) is correct. Therefore, it is not permissible for you to travel without a mahram except out of necessity. Many scholars have permitted a woman's travel without a mahram for an obligatory Hajj or obligatory Umrah (definition), provided she is with a trustworthy group. Some of them permitted her travel without a mahram if the travel was for an act of obedience and she felt safe for herself. However, it is preferable to refrain from traveling without a mahram absolutely, except out of necessity, or for an obligatory Hajj or Umrah, and this is the view of the majority of scholars.