All depends of what version of Ubuntu you are running. If you have the latest 16.04 that ships with postgres 9.5 then you should try by installing postgresql-server-dev-9.5

Otherwise try an apt-get update to refresh the list of available packages

For that open software center in ubantu :

In that Edit -> Software Sources -> Ubuntu Software tab, in that from "Download From" option you can set server of different location/ country.

Hope this will helps you :)