It's a little wierd to use such a thick block of metal for anything (except making a die or something), so you may want to rethink your design, whatever it is.
I would definitely check eBay first, because it will be a LOT cheaper than going to a metal shop. BTW the kind of dealer you want is called "non ferrous".
Some places will sell off cuts and scrap by the pound and once again that will be a lot cheaper than buying it custom. You do not say where your location is in your profile, so not sure what dealers are in your area.
As Jay says, get a pro to cut it, because setting up to cut metal, while not terribly difficult, sounds like it is beyond your skill level right now.
As for the type of aluminum, I wouldn't worry about it for something like this.