My ISP has just bloked port 25, so I need to find a way to use a different port.
No, you need a way to tell the whole rest of the world to use a different port to send you mail. And there's no way to do that.
You'll need to have your domain's mail delivered somewhere else, where they will store it for you until you collect it, or they will need to deliver it to you via a non-standard port.
Or you need to buy an Internet connection which doesn't have port 25 blocked.