The NPK ratio is printed on the front of all plant food boxes, jars, can, jug etc....
Simple plant food can be boughten from WalMart, Home Depot, Ace Hardware etc...
If you're looking for better results, hit your local nursery for better brands of plant food, or just do some reading on what you think will suite you best and buy online through ebay or something. Everything can be found online.
i dont like wal-mart or local stores mostly they sell miracle grow. google a hydro shop near you, they will have all type of goodies and can help with your choice. you can go pretty cheap with dyna-gro or genergal hydroponics, etc...the amount of npk will depend on strain.