Plan b pill

So I had Sex a few hours ago and we didn't use a condom cause he thought I was still on birth control but I'm not and I was just in the heat of the moment so I didn't get to tell him not to cum inside of me. And my calendar says I'm fertile so I need to take the after pill. 
Does anyone know what store the plan b pill is cheaper ? 
What are the chances of it working if I take it tomorrow morning ? 

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