mamaLori is right- I buy the Costco sized bag of Ghiardelli's and add 1/4 cup oil to it.
To prep it you need to put all the chocolate chips and oil into a microwable bowl (I prefer pyrex as it's easier to clean than plastic or tupperware). Microwave it in 30 second intervals, stirring at each interval. Make sure that you do not get any water or watery liquid on your stirring spoon (ie do NOT wash the spoon in between stirs) as it will make the chocolate "break" and it will not be smooth.
It usually takes about 10 minutes of 30 second intervals to melt the whole thing. Once it is fully melted, turn the fountain on to "warm" and pour the chocolate in. After a few minutes you can turn it on an viola, chocolate fountain!