Supermarkets used to carry the small cubes of fresh yeast but in my area (outside of Dallas) they are no longer available. You might check your local supermarkets to see if they have the product. Fleischmann's was the brand that I used to see. For a photo of that product, see the opening post at http://www.pizzamaking.com/forum/index.php/topic,10947.msg98071.html#msg98071
. Typically, the yeast cubes are stocked where the margarines and butters are kept. The supply is generally minimal (the demand is low) so you have to look carefully so that you don't miss them entirely.
Another option is to find a local baker who uses fresh yeast. Unless you need a lot of the fresh yeast, they may even give you some for free. Unfortunately, many professional bakers have also switched to dry forms of yeast.