They're standard 12mm hexes. You can get them from any hobby shop.
Yes, hexes are cheap. $3-$4 usually for 4. Stick with the plastic. They're cheap, easy to replace, and won't strip out your more expensive wheels. Aluminum is not always better, in fact a lot of parts are better off in plastic form. Plastic will flex. Aluminum will bend. There are certain things that benefit from aluminum, just make sure you want it metal for a reason.