Right. I found the 7805, but I can't change the voltage on it - which actually makes sense, because if you changed it to 12 volts, it would no longer be a 7805, but a 7812. I just can't find a 7812 anywhere - which is, again, amazing, as I would have thought that that was such a basic part that it would have been added, long before moving on to other, more advanced stuff.
It's like buying a car and finding out that it doesn't come with an engine, you have to make your own.
I have no problem with building a part, but have no idea how to do that. Is there a tutorial somewhere?