I tried to google it for you, and you're right, the availability of parts is next to nill. I did find one reference of a
Model 72 for sale at auction in Minnesota, but what was significant was the description:
HERE IS THAT WINCHESTER THAT WILL BE GREAT TO START A YOUNG CHILD ON HIS OR HER SHOOTING EXPERIENCE.THIS IS NO COLLECTOR IT HAS BEEN WELL USED.THE BOLT HANDLE WAS BROKEN AND RE WELDED.AND THE TOE OF THE STOCK WAS BROKEN AND REPAIRED PHOTOS SHOW THE REPAIR GREAT REPAIR.REAR SIGHT ELEVATOR IS GONE EASY FIND.BORE IS SUPER SHINEY IT'S JUST A SUPER NICE PLINKER.NO CALIFORNIA SALES.THIS ITEM SOLD AS IS .
That auction was from March, by the way, and is closed. But from both the auction description and the complete lack of availability for that part, it would seem that busted bolt handles on Model 72s was not uncommon. My suggestion would be to take it to a gunsmith and see if he can't weld on a bolt handle of appropriate size and shape. If you don't mind that it's not a factory original part, then at least the rifle would be returned to serviceability.
I found this picture of a Model 72 bolt from a
Model 72 at auction on gunbroker.com, with a "Buy Now" price of $1,250.00 (!). I guess they are pretty collectible.

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