It is very hard to use a voltage source to induce charge on an insulator. The reason is that by definition, an insulator does not conduct electricity - so if you apply an electrode at one place, you will not move electrons elsewhere, and so you cannot induce a net charge (the best you can hope for is to create polarization, and maybe pull off a handful of electrons at the point of contact).
People make high voltage generators (for example, Van de Graaff generators) that are used to charge conductors - for example this one which you can buy from xump:
A large voltage (up to 350 kV) is generated by the moving insulator band that runs up and down the insulating tube on the left, and a differential charge is induced on the two conducting spheres. When the potential difference is large enough you get a nice lightning bolt. But this only works because the charge can distribute itself along the surface, which is conducting.
The induction of charge on an insulator happens when you "steal away" electrons one at a time. When you touch two dissimilar insulators against each other (say silk and glass), then the molecules of one tend to have higher electron affinity than the other. As a result, when the two surfaces are separated, it is possible that an electron is transferred from one material to the other. Rubbing is a way to do a large number of "stick-unstick" operations millions of times per second, and this results in charge buildup. And since the insulator does not conduct electricity, when electrons are "stolen" from the surface, they do not get replenished from elsewhere. This is a fundamental difference between charged insulators and charged conductors.
Finally - the relationship between voltage and charge is given by the capacitance of the object. For a sphere, the capacitance, usually denoted by $C$, is given by
$$C = 4\pi\epsilon_0 R$$
and the relationship between voltage $V$, charge $Q$ and capacitance $C$ is given by
$$ Q = C V $$
So for a sphere of 10 cm radius, $C = 11.1 pF$ (picoFarad), and a voltage of 350 kV (above generator) would induce a charge of $3.9 \mu C$ according to the formula.
For larger spheres, the charge would be larger - the answer scales linearly with both radius and voltage. But it's awfully hard to get a measurable amount of charge this way.
It is possible to charge an insulator without rubbing. Imagine the following setup:
A conducting sphere coated in an insulator. Apply a large voltage to the sphere. This results in an electric field normal to the surface of the insulator.
Now water molecules in air are partially ionized. The molecules with the appropriate charge will be attracted by the sphere, and on contact will neutralize, taking a bit of charge with them. This creates an apparent "leakage current" and results in surface charging of the insulating sphere. Note that in this case the charge is NOT coming from conduction through the insulator - it is coming from the air. It would require an experiment in a good vacuum chamber to prove whether conduction through an insulator can result in charge buildup. The answer is "probably, somewhat". But it's not a good mechanism to rely on for your experiments.