Much though I hate to be the one to say this to you, I think your answer is that most likely, "it ain't gonna happen". The reason that I say this is that in order to make a plasma ball, you have to build a small contraption and enclose it in a glass bulb that has been evacuated and filled with a special type of gas. Getting this gas and enclosing it in a glass bulb along with the working part of the plasma ball would most likely require a lot of specialized equipment that you probably can't get at all, and if you can, certainly can't get cheaply.
But don't give up hope yet! Plasma balls work by building up a lot of electrical charge. When the electrical charge goes flying off through a gas, it lights it up. The amount of electricity used by a plasma ball is not enough to light up air, but it is enough to light up the special gas that it uses. But if you build the same sort of thing, only with a much higher electrical charge, you get something that can
light up air, a Tesla coil:
Making a Tesla coil is not a minor project, but it is a lot more realistic than building a plasma ball. But because they use so much electricity, you do need to be very safe about doing it and make sure you know exactly what you're doing before you start. For more detailed information, you can check out this link on the Construction of a Tesla Coil
to get started.
p.s. Since writing this, I have found a nice site on how to build a /relatively/ simple plasma ball at home. For more information on that, go to http://www-astro.physics.uiowa.edu/~rnw/globe.html#Light
. Again, good luck!
(published on 10/22/2007)