Just use regular ice. Some people have done that with good results. I did it on my last brew and actually had too much ice (the wort reached 70F in about 20 minutes and still had a small iceberg in it).
I froze about 0.75 gallon of water in a plastic 1g milk carton, and then cut off the carton to get the ice out. I had about 3.5g of wort to cool, and also put the pot in an ice bath. The ice block was too much. Next time I'll try a 0.5g ice block, or split 0.75g into two separate containers. (multiple smaller chunks of ice will cool faster than one big chunk of ice).