As far as alum goes I would just look around to some metal supply houses, Admiral would be one. Stuff is not real cheap though. As far as scrap yards go I would think it would be hard to stumble across a nice piece of 1/2 thick alum. the size you need but you never know. I'd go with 1/2" min. thickness.
Admiral Metals
for info if you are going to use an aluminum base plate and are going to put an aluminum sled on top of it to hold your cutting tool you will need to put a plastic bottom on the sled as alum. to alum. will gall up in a short while, thus no smooth movement. I mounted a thin piece of polyprop. on the bottom of mine. Works perfect. You can use 2 sided carpet tape to accomplish this. I always thought a piece of Corian counter top material would make a good base plate. Also don't forget to get your base up as high as you can so your sled, tool, will not be too high off the plate, which makes it way too tippy , unstable, and harder to control. You are controlling all the movement by hand so you want it as stable as you can get it or your wrists will fatigue pretty fast, especially with hard wood species, plus it will be scary
