You can make your own nice juggling balls several simple ways:

TENNIS BALL METHOD

1. Make small slits into tennis balls
2. Fill them with rice, beans, I even had a friend who used about 20 pennies in each (a bit noisy)... Whatever you put in, put the same amount in each ball for uniform weight.  It’s  useful to note: If you don't fill them quite all the way the balls tend to stay nearby, rather than roll away from you when you drop them.
3. Then tape them up with colored electrical tape. Stretch the tape just enough so that it goes on smoothly, but not  more, or the tape can move about a bit leaving sticky glue exposed.

An alternate tennis ball method:

Use a syringe to fill each ball with water. Pay close attention to getting the same amount of water into each ball, as a consistant weight makes a big difference, especially when you are juggling 5 or more balls. The hole is small enough that the water does not escape, and no sealing of the ball is necessary.

BALLOON BALL METHOD (my favorite)

This creates a nice completely smooth ball using 3 balloons (12” size) filled with 3/4 cup (or adjust for the size you like) with short grain rice as follows: You’ll need:

• A measuring cup
• A bottle (I like the Snapple size bottle because of the wider mouth it’s easier to fill with rice)
• 12” balloons (4 times as many balloons as you want balls)
• Short grain rice (3/4 cup for each ball you plan to make, plus a little extra cause it's easy to spill)

Sit outside, or in an area where it's okay to spill some rice.

1. Pour the rice from the measuring cup into the bottle.
2. Inflate the balloon with just a couple of breaths (so it’s bigger than a juggling ball). and twist (don't tie) the neck of the balloon, and stretch the mouth of the balloon over the opening of the bottle.
3. Let the balloon neck untwist (You now have a partially inflated balloon on the neck of  a bottle of rice.)
4. Invert the bottle with the balloon and pour the rice into the balloon.
5. Take the balloon off the bottle, and let it deflate around the rice inside.
6. Cut off the entire neck of the balloon, leaving a small hole with rice exposed.
7. Take a 2nd and 3rd balloon and cut off the necks - plus a little bit more, so the hole is bigger than the neck
   [Option: cut tiny holes in 3nd balloon...This makes for very colorful balls, you can even cut tiny fancy patterns of holes if you're patient. This does make the final stretching in step 11 more difficult, but every hole will leave a poka-dots of the underneath color showing through once it's stretched over] (illustration step 7)
8. Stretch the second balloon wide open (it will sometimes break while trying to do this) and, covering the small exposed hole, enclose the rice-balloon with the 2nd balloon. (steps 8 and 9 in illustration)
9. Turn the ball over
10. Repeat with a third balloon and stretch over the ball , covering the place where the ball is just one-balloon thick.
11. Tada!