Find your REAPER resource path by choosing "Options > Show REAPER resource path in explorer/finder..."
Locate the 'Cursors' folder.
Valid .cur cursor files placed within that folder will override the default cursor of the same name. A REAPER restart is required to enable the override.
arrange_ibeam:
Time selection. Defaults to I-beam system cursor
arrange_itemvol:
Media item volume adjustment when the preference 'Adjust media item volume by dragging' is set to 'top edge of media item'. Defaults to 'double headed vertical arrow' system cursor.
midi_bg:
(Mouse Modifiers > MIDI piano roll > left drag > Default action > No Action) Left dragging in empty space will do nothing. Defaults to arrow system cursor.