I believe that you can "unscrew" the tang from the handle.
The best way to find out is to contact Helle Knives, and pose the question to them..
You can get to them by this email,
[email protected]As the sheath is tailored to the dimensions of the handle, I don't think you can reuse it.
For leather you can always try the big auction site, texas knife company or Tandy Leather, whom I believe many here use!
Good Luck, and do show us how it turns out