This topic has been discussed many times in different forums and had you searched in Google, you would have got lot of inputs from there. Please go through the forum rules which you can see to your right screen and try to adhere to that and avoid posting repeated queries like this. For information, when I checked in Google with the text "automatic po against sale order", I could see some 20-25 URLs.
Though you had mentioned that you searched, please share those links and explain still what you want from SCN.
G. Lakshmipathi