Why are you looking for the cheapest place? In terms of nail polish, it's often a matter of a few cents, and you waste more in gas or bus fare to get to a store that may sell it for only 10 cents less. Just check your nearest pharmacy.
onine would be the best place in my opinion. you can buy more than one bottle or a couple different things to make the shipping worth the cost. :)
The mall?
Kijiji.ca