Some to try are;
Handyman Steel Supplies, 29 Nellbern Rd, Moorabbin.
Lockbar Steel & Aluminium Supplies, 2 Barlow Ave, Cheltenham.
As far as aluminium requirements go, most dedicated aluminium suppliers should have shorts or offcuts available. I often use Capral Aluminium, they appear to be in a new location now, 41 Greens Rd, Dandenong. I mainly buy flat plate offcuts, can't recall the exact grade, but it's very tough, excellent to use, available from about 4 to 150+ mm thick.
I just reread your previous post, you may find the grade of aluminium in the plate form ideal for engines, rather than mild steel.
Paul.