Bought mine from motorcycle accessories supermarket ( MCA ) on Wentworth ave in the city.
They have a good range and will come out to your scooter to ensure the mounting is compatible.
2 years ago, I paid $75 which was more than I anticipated. But it's a quality brand called GIVI , has been fully waterproof and it's stood up to the constant attachment/de-attachment. Might be a bit cheaper online, but at the time I didn't have time to wait for shipping.
I run a cable to a powerbank in my jacket pocket for charging. Don't know if cases with a battery compartment are out there. If you do come across one, have a good feel of the combined weight, as anything to heavy may cause the mounting bracket to constantly rotate downward.
Heads up that if you do visit MCA get there before 3pm for parking, as outside the clearway starts.