Where are AWS EC2 availability zones exactly?

You can assume that all availability zones in the same region have the similar latency. (as they are all very close). Also as far as I know not necessary the availability zones are the same for all users. (i.e. us-east-1a can be different datacenter for different users)

The best that Amazon provide in terms of public information is on their EC2 FAQ. If you want the lowest possible latency can you not just ping servers in each availability zone from your target location and find the lowest latency that way? There's no guarantee that physical proximity == lowest latency anyway.