How can I find the depth of a portion of the Valles Marineris? How can I find the depth of the low lying part of this image of the western part of Valles Marineris?

I'm interested in the area northeast of Noctis Labyrinthus, southwest of Tithonium Chasma, north of Oudemans, and west of Ius Chasma.

I couldn't find any contour maps of Valles Marineris or Noctis Labyrinthus
You can find MOLA laser altimeter data here:https://pds-geosciences.wustl.edu/missions/mgs/megdr.html.  The files are simple rectangular grids of differences between Martian "sea-level" and measure height at 1/128 degree resolution.  If you have the latitude and longitude, it is quite straightforward to find the height.

To view these files, you probably need the software found here: https://pds.jpl.nasa.gov/tools/about/pds3-tools/nasa-view.shtml 

Looks like the software might be a little out-of-date, but you can give it a try.