Home Depot, Lowes and some hardware stores have racks of rod, angle, and
strip steel that might meet your needs. Plus there is pipe, bed frames, and
the nearest metal scrap yard.
If you get desperate, you can buy damn near anything online.
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In that case, try Potomac Steel, off Loisdale road just a bit
south of where I-95 hits the Washington Beltway (I-495).
Either be prepared to pay for cuts (if you want shorter pieces),
or to buy full-length pieces and cut them once you get them home.
The times when I have been there, they have been reasonable
You'll get better steel than you could get from Sears, or from
Home Depot -- but you may have to make choices. Since I don't weld,
I'll leave it to others to suggest reasonable grades of steel to buy for
the purpose. I like 12L14 for machining, but it is supposed to be very
bad for welding, and I'm not even sure that they stock that. I've been
lucky enough to get in on some group purchases as part of CAMS
(Chesapeake Area Metalworking Society).