You can get that cheaper by building it yourself:
- 4 x 1.2 V rechargeable batteries (AA, Baby or Mono, depending on which capacity/weight ratio you prefer)
- A case to hold the batteries
- a plug for 9V batteries (to be attached to the battery case)
- some cable
- a plug that goes into your Z.
Beware! Never replace the rechargeable batteries with 1.5 V batteries, as this would blow your Z (6 V vs the 5 you need).
Good luck
Peter