Any of your Electrical supply houses will have it. there is a letter / number combination on the plug that you will need to bring with you to get the correct plug. It will go something like NEMA - 5-15P. that will allow the person at the counter to quickly find the match to your plug.
Sometimes you can find them at the Big Box stores also, but unless you are made of money don't go to a Marina or RV store, way over priced IMHO.
