Really simple to build your own.
They're a lot of fun and really cheap to make.
Step 1: String and Ball
Start with finding a good piece of string. I found mine in the basement, it's a synthetic string that might have been for clothing line or something. Just make sure it doesn't break.
For the weight I use a tennis ball. Just the right weight, cheap and easy to find. I wiggled a screwdriver through to make the holes. Make sure the holes are opposite or it will wobble. I used thin metal wire to get the string through the ball. A metal washer or two makes sure that the knot doesn't eat through the holes.
Step 2: A Little Bonus Feature
To be able to change the length of the string I used one of these wire thingies (don't know what they are called). Shouldn't be more than a dollar or two at the hardware store.
Step 3: Handle
To make the handle I got who key straps from a telemarketing guy handing them out in the park. I cut it and folded it around the metal link and sowed in the ends with a sewing machine. I had some problems making it stay in position in the machine, but I nailed it to a piece of paper and sowed right through. When done just tear the paper of. I used a synthetic thread and ran across a bunch of times with a zigzag stitch.