Various types of Ice Ball Mold options are available for online purchase from this source. The one seen here is just one example of many. See them all through the following link:
Browse all Ice Ball Molds here.

This Ice Ball Mold is the Stone Cask Ice Rounds. Makes 2 large ice balls to chill your drinks. Close it up and pour water into the hole at the top and let freeze.
Read more about them, or buy them here.

Pros: 

Easy to use and remove ice balls. Dishwasher safe. All kinds of great customer reviews to read through. Many other Ice Ball molds than just this set. Check them all out.

Cons: 

Read all reviews good bad and indifferent through the provided links.

This one is the Tovolo sphere ice mold maker. This is a set of two. They are designed to stack up to save space in the freezer. They also make a version designed to freeze cubes. The caps are silicone. They are also designed in such a way that they will not leak or tip over in the freezer. Dishwasher safe and BPA free. Click the image of the stackable ice ball makers shown here to find a lot more details on this product and how to buy them.

Make ice ball molds a lot more efficiently by making four ice balls at a time using this mold. This one is called the Rox Sphere ice ball mold. The way it works is with the two piece mold device. One piece is the bottom that you see pictured here. A top piece is placed over this one. The top piece creates to ball shape and has a hole in the top. The hole in the top is obviously where you would add the water (or any other type of liquid you wish to freeze into a ball shape). The bottom piece has feet to hold the device flat and secure to avoid tipping over and spilling. Click the image to read more details, see more camera angles, or read customer review comments. You will also find more information on how to buy these.