Usually they are available at any service station with a big picture of one out front and cost $35.
We can assume you already checked Walmart and did not find one anywhere. So now you have to go underground.
News flash, nobody is going to be dumb enough to sell them online. Plus, I hate to be that guy, if the police run the plate it tells them if the car was inspected, they dont go off just the fact that there is a sticker in the window.