The list of IP addresses from which Amazon SNS notifications are sourced is provided in the Amazon SNS forums. We update this post as the IP information changes.
Please note that while notifications will usually be sourced from an IP in the same region where your topic was created, this is not guaranteed e.g., a notification sent from a topic created in US-EAST-1 may be sourced from either US-WEST-1 or EU-WEST-1. In other words, you should add all addresses on this list to your ingress rules, even if you only plan to use a single region.