A new 915 head that is from Titleist and not a knock-off will be extremely hard to find. Your best option would be to search used club sites that offer an authentic guarantee and list the condition to be excellent or near excellent. 3Balls and 2nd Swing are 2 I've used in the past. There are more chances on eBay to go wrong with some of the more obscure sellers there.

You may want to have a shop inspect the head. Other than a crack, I've not seen a head go dead from wear. If not cracked, you may not benefit from a replacement.