Ditto to what @rbtdanforth said, I’ll add that acrylic is pretty standard across all industries. You’re look for cast acrylic (if it’s marked for a laser it’ll most likely be cast), and with the 
you need to avoid clears, whites, and blues. Keeping that in mind you can get it from nearly anywhere 
If you can find Baltic Birch plywood (not just Birch, but specifically Baltic) that’s made with a wood core and 1/8" should do just find in your Aura. You can look online, but also if there are any cabinet shops in your area they’ll often have “cast offs” that they’ll sell for cheap. I once got a whole box at a Habitat for Humanity ReStore for only a few bucks a sheet! I’m still working my way through that.
Keep in mind that one of the reasons the Proofgrade 
is comparatively expensive is that they literally guarantee it’ll work. If you use the correct settings and it doesn’t work, they’ll replace the sheet. If you’re using random work you’re going to need to spend time figuring out what settings work (BTW, my default for any wood to start with is the Basswood settings, it works 75% of the time), and there’s always the chance that there will be voids, or they’ve used glue that can’t be cut by a laser, or a dozen other things that cost either time or money. Something to keep in mind, especially if you’re planning on trying to do any sort of commercial venture (though the is definitely not designed for commercial use!)
The one thing that it excels it (it and the Spark) is papercraft. Cutting cards, and layered art, and foldable things using cardstock is an incredible rabbit hole to fall into, and it’s the best machine for that!