How to get the list of fleets available across all the AWS accounts.

This is not something that is natively supported. You would have to look at each account.

You could look at your consolidated billing to narrow down which accounts have AppStream 2.0 usage.

You can also look at each account programatically. I recently found Botocove that lets you run a function against a selection of AWS accounts / OUs. This is not written or validated by AWS, but might meet your needs. https://pypi.org/project/botocove/