In case there is a need to view all the available snapshots from all of your account, for all of the availability zone using one command, this is how to do it.
create the owner-list.txt file which contain all of your AWS accounts number, each account should be in a line, for example
# cat owner-list.txt
then create az-list.txt file, with the following availability zones
# cat az-list.txt
now create the script itself
while read az; do
while read owner; do
aws ec2 describe-snapshots --region $az --owner-ids $owner --output json > $owner.$az.json
the output will be list of json files, for each AZ and account.