Mild Steel (CR/HR), Stainless and some aluminum:
Mill Steel: 62 Maple St, Manchester NH 03103. 603-626-7351 - Open 7:00-4:30 M-F
Mill carries most sheet/bar/stock. They do not carry much aluminum but have basics like 1x1 tube.
They're prices are reasonable and are very happy to show you drops.
Remember, new sheets come in 4x8 lengths and most bar/round stock in 20-24' lengths.
They will do the first cut free, making it a lot easier to transport. Each additional cut is $10.
See attached millsteel.pdf for stock availability. (Updated 9/2015)
Yarde Metals: Yarde Dr, Pelham, NH 03076 (603) 635-1266
Yarde is great for aluminum. I've yet to order but been told that they're fine on single sheet/piece sales.
FYI: Yarde also sells stainless and regular steel, but they're buying it from Mill, so you'll be paying more.
Choice Metals: 36 Cote Ave, Goffstown, NH 03045 - http://www.choicemet.com/index.php/products
I have yet to visit but am told they are great at getting odd pieces. If you need a 2x2 sheet of 3/8" 6061, they'll have it or be able to find it without requiring you to purchase an entire 4x8' sheet.
Marmon/Keystone: One Clark Street Southampton, MA 01073-9354 - www.marmonkeystone.com
If you need structural steel, these guys generally have the best prices on DOM. They do not make cuts. Tube/bar stock will be anywhere from 18-24' long. Will deliver to Makeit labs for a reasonable fee.
Metal Supermarkets Boston (North): 16A 6th Rd, Woburn MA -- (781) 933-0176
Cold and Hot Roll Steel, Stainless, Aluminum, Brass, Bronze, Copper, Tool Steel. Gary at the front desk is very helpful. They have a hobbyist section (cuts, ends, etc. ask Gary for a tour) and have no problems with small orders.
Granite State Salvage: 31 Dracut Rd, Hudson, NH
They allow you to walk around and let you fill up a container, weigh and buy.
Scrap Metals Inc: 128 East Glenwood St., Nashua, NH - (over by Rivier College):
He's got some sorted and big, big piles of mainly steel, some stainless and aluminum. There are also random machines and parts there. It's a mess, but some things are organized. Wear old shoes/work boots and jeans. Ansel will let you walk around (cannot climb piles) and find what you want. Once you know what you'll need, make a pile and someone will grab a forklift/pallet to weigh it. Price is $0.35/lb for regular steel. Talk with Andrew Ryder before going here because he knows the owner and can usually get better prices.