If you know how to compile blender, then you’ll find all the cursors defined in wm_cursors.c. There are comments in there describing the format that you need to use to edit or replace them.
Do you have something particular in mind to replace that crosshair cursor? Keep in mind that they can’t be very complex. In a nutshell all those custom cursors are a grid of 16x16 pixels, each of which can be transparent, black, or white. There is an option in Preferences to show large cursors (32x32) but most people would use the small ones.