YT USA has 540Wh spare batteries in stock online right now.
https://us.yt-industries.com/products/parts/decoy/mk1-2019-2020/288/540wh-battery-set-decoy-mk1-mk2/
I emailed The Mill (YT USA) about a waiting list for a battery, this is the reply:
"Thanks for reaching out!
We unfortunately do not have a waiting list or pre-order list for new batteries. All bikes and parts are first come first serve.
We do have a large re-supply of batteries on order and we...