Case fans can be picked up just about any PC store and I would imagine Ebay would be your best bet for the CPU heat-sink. I have,not so long ago anyway, thrown out a few as I cannot remember when I last fitted one. Shame as i would have quite happily sent you one. If you do have problems getting hold of a suitable fan/heat-sink let me know.
Dell XPS 13 9370 (2018) review