• Bullet Head nails from Bunnings work well
• 3, 4 or 5mm diameter mild steel round bar from a steel supplier. This is a softer iron alloy, so it will rust more readily. Very cheap.
• Black rod ties, for rebar
• Zinc plated, non-galvanised nails from Bunnings rust the best.

I would suggest using fine steel wool, as most steel nails are not pure steel anymore and are also coated in oil to prevent rusting.
Roll the steel wool into tiny balls and use in a test tube, if you get rust stains in the test tubes after the experiment, just rinse with a little concentrated HCl.

I got normal nails and soaked them in acid to remove the plating.