Welcome to the site !!
I would think , you would have the best luck on EBay , and for future protection , you know those Aluminum guards you always see on 2 stroke pipes ?? , they now have them for 4 strokes , and are fairly inexpensive , it might help prevent another ding when you find a replacement
https://www.ebay.com/itm/DRC-4-Stroke-U ... 0#viTabs_0
As far as the header goes (I just searched EBay and found nothing) , not sure how bad the dent is , but have you tried freezing it ?? , you put water in it where the dent is , then stick it in the freezer so the dent is down , so the water is in that area completely , then let it freeze , keep checking it , and see if it pops the dent out ....worth a shot