Can I just source my own replacement now I have moved out and deliver it to the house?
Unfortunately not. You are allowed access to the property (either yourself or via a third-party) during the dates of your tenancy. After you have vacated the property, any access or works need to be arranged by the Landlord or Coapt.
I have been charged for a missing item but I am sure it was left in the property, maybe in a different room or a cupboard?
The check-out clerk will thoroughly check the property and list any items that appear in different rooms compared to the inventory. A staff member from Coapt will also visit the property the day after you vacate, as well as the day before the next set of tenants are due to move in, and will ensure that you are not charged for items that are recovered.
Why am I being charged when the landlord has no intention of actually replacing the item?
If an item is listed in the inventory but missing from the check-out report, the landlord has every right to charge the cost towards replacing the item although they do not actually have to replace it, as it is an asset they have lost.