You can't just find a script and just set it up,

You're going to need to hire developers, a support team, maintainers

An exchange is no good if no one maintains it and it's just a script, no script can automatically choose and add new coins from the altcoin announcement section of bitcointalk without having many security flaws

Either you need to do the above and literally start a new company, or be one insane genius developer who will spend most of their day maintaining the exchange to make sure there are no flaws in your system, your customers are happy and you can keep up to date with the market and withdrawals/deposits.