Is it possible to remove the status of Niddah without a mikvah? I've heard from other people you can remove certain levels of impurity with a shower and possibly other ways (no sources though) so I was thinking there may be other alternatives for people who are Niddah and don't have access to a mikvah.
Shulchan Arukh (YD 201:1) writes:
אין האשה עולה מטומאתה ברחיצה במרחץ ואפילו עלו עליה כל מימות שבעולם עדיין היא בטומאתה וחייבים עליה כרת עד שתטבול כל גופה בבת אחת במי מקוה או מעיין שיש בהם מ' סאה.
A women does not remove her Niddah impurity through washing in a bathhouse, even if all the waters in the world washed over her, she remains in her impurity and one is liable to Karet for [intercourse with] her, until she dips her entire body in a Mikvah or Maayan that has 40 s'ah volume [of water].
So there are two options: a Mikvah, which is a stable collection of undrawn water of minimum volume, or a Maayan (spring), which is flowing water connected to an active spring. You can think of these as two different kinds of Mikvaot which have slightly different construction considerations. Dipping in either fully removes Niddah impurity.
If a modern constructed Mikvah is not around, often a Maayan is, such as the ocean (which is connected to springs underneath it).
Be sure to speak to a rabbi for a ruling about any particular body of water as the details here can be quite complicated.