Do you want to construct a variable power supply? i.e 6v, 9v, 12v?
You could use a transformer say: 240(mains)/15v and regulate this to 12, 9, and 6 volts using the appropriate regulatros i.e 7812, 7809 and 7806.
What's the current rating? I know of transformers capable of a maximum of 0.5A, 1A, 3A etc. I believe these should be available. To make a variable PS you might need a sort of switch to switch the transformer output to the desired regulator (just my own idea).
It all depends on what you want. Cheers!!