For using JSONP the other server must provide an interface to it. For example: The other server must provide an URL like
http://other.com/getvalue?jsonp=callbackFunction
This URL must sent JavaScript code like
callbackFunction({ value: "Get Me If You Can" })
If the other server does not provide such an interface and do not provide the Access-Control-Allow-Origin
Header necessary for Cross origin resource sharing you cannot load the other site via AJAX.
But you can always program a crawler which stores the value periodically on your server (where you can use it). Please respect the other server's robots.txt in this case.