The "Boom Cubes" seem to have to do with the cubic patter design on the fur of the Furby toy. That seems to be the only difference on the toy in this case.
toy stores
Toy balls are available at toy stores and most major retailers. In addition larger grocery stores, particularly in the summertime, may very well carry larger balls.
string balls, toy mice or anything that sqeaks
The character Bucky O'Hare is from many comic books, the first one was Echo of Futurepast in 1984 published by Continuity Comics. This character has the video games and toy lines.
They can, but it would be better to get them a toy designed for horses, as balls and other human 'toys' are not strong enough to hold up for long.
Amazon currently stock a range of Furby Boom Cubes toys with the pink toy costing $56.96 and available for free shipping. You should be able to buy one from a large toy shop such as Toys R Us.
balls
One can purchase toy balls through several different online retailers. Some of these include: Amazon, Toy Wiz, Toys R Us, Splat Back, Best Buy and Target.
Powder Toy is better.
Dogs. Toy poodles!
There is one simple purpose for balance balls. This invention is for exercising. However, some people use it as a toy even though it does not make for a great toy.