If you are expecting your period before your trip, but haven't gotten it yet, please contact your physician. It may be possible to medically delay menstruation so that you can remain tehorah while you are away.
Otherwise, if there is no mikvah available at the time you are scheduled to immerse, unfortunately you must delay tevilah until you have access to a mikvah. Immersion may not take place early, since the seven clean days must be complete in order for immersion to be valid.
If you wish to share more details of the natural body of water that is close to your destination, we may be able to give you suggestions on how to immerse safely. Alternatively, you can try to contact Chabad in that general location to see if they know of another mikvah option.
Please feel free to get back to us with any further questions.
B'Hatzlacha!