Columbus Pipe & Equipment for sheet scrap. They have a few 1000 sf. of floor covered in random pieces. It's all cut into square/rectangular pieces. 14ga to 1" thick, 6" square to full sheets. They also have pretty good prices carbon sheet.
Research alloys for random stuff. They stock some square tubing and sheet goods. They are very reasonable. Good prices on stainless sheet as well.
Benjamin Steel if you want to order and have it delivered and have a forklift/business address. I pay $9.50 for fuel surcharge - less than it would cost me in gas to pickup myself.
I haven't used Alro as their prices are insane unless you need 100 tons. I browse their site as a baseline and if I need something specific.
If you're near Plain City (Industrial Pkwy.) and want miscellaneous scrap to practice on, let me know.