You should firstly take it to the mechanic who did this and then get kick on his behind and let him correct the issue...
Bhai any good mechanic can do this. And this packing is available OEM spec at waterpump why are you going to plaza. Go-to to SuperWay Autos at waterpump just beside Mehmood Sweets and ask for the thing. Better take the old one that was taken out with you.
I am still not understanding why it leaked again if it is replaced in the morning... First the engine now the transmission...
If you are visiting plaza it is available at Japan Autos.