Tom,

Your cheapest bet for good casting material is to use sand-cast
aluminum scrap such as cylinder heads, transmission housings,
differential housings, etc. of OLDER cars where these were indeed
sand-cast.

You can usually tell a sand casting by its characteristic rough surface
where unmachined. Unfortunately modern cars have most of their drive
line housings made of die castings, which are less suitable for
subsequent sand casting.

Drawn material such as tubing, wire, architectual extrusions, etc. will
be very soft after casting since these alloys get their strength
through cold drawing and/or subsequent heat treating. This is a big
subject.

If you stick to basics as described above you should be OK. Try to get
some degassing tablets to reduce porosity thus improving structural
integrity.

Hope this helps a little.

Wolfgang