Amazon EBS snapshots are stored in Amazon S3. Data stored in S3 is replicated across multiple devices in multiple facilities. (See Amazon S3 FAQ.)
While Amazon S3 defines multiple storage classes, there is no specific mention of the storage class used to store EBS snapshots. However, it would be a fair assumption that it uses Standard Storage (3 facilities, instantly available, no retrieval cost) to ensure high durability of your data.
EBS Snapshots are stored at the block-level and are kept in Amazon's own account rather than your own account. Therefore, you will not see snapshots within the Amazon S3 console; it can be viewed from the Snapshots section of the Amazon EC2 console. Therefore, you have no ability to influence the way in which it is stored.
The cost of EBS snapshots is also independent from the normal cost of Amazon S3. Fortunately only used blocks are stored, data is compressed and snapshots are incremental, so the cost of a snapshot is minimized.