Before Passover, the Kosher-for-Passover Coke (and Pepsi) will be in the section of special Passover foods. Typically, most supermarkets carry 2-liter bottles. You can sometimes find cans at kosher specialty stores.
In Chicago, the bottle caps will be marked with the triangular CRC logo of the Chicago Rabbinical Council, and it will say P05 or Passover 2005 somewhere. On cans, this is sometimes on the can and sometimes only on the flats.
Usually, the Passover Coke costs more than regular Coke.
However, at regular markets, after Passover, any k-f-p Coke left usually gets mixed in with the regular product on shelves, so if you're sharp-eyed and lucky, you can find it at regular price.
If your supermarket doesn't have a fairly good stock of other Passover products, you probably won't find the Coke, either.
By the way, not only does the sweetener change, the carbon dioxide does, too. It has to be made from nongrain sources. I rabbi I once spoke to about this said, "Well, you may say it's just a gas, but I ask you, would you drink Coca-Cola without that gas?"