When the inventory of RDS instances in the target Availability Zone is insufficient, you have the following options:
If the business needs are not urgent, please wait for a while and wait until the target availability zone has instance inventory before purchasing. However, due to the different conditions of each supply chain, the time required to replenish inventory cannot be determined.
If the instance inventory in all availability zones in the same region is insufficient, please purchase an instance in the region closest to your server. After the target availability zone is replenished, the instance is migrated to the target availability zone. For details, see Data Migration Between RDS Instances. However, please note that instances in different regions need to communicate with each other through the external network, and additional charges are required.