The best place for true scrap metal in Seattle is :
West Seattle Recycling Center
3881 16th Ave SW
Seattle, WA 98106
Steel, aluminum, lead, copper, stainless, iron - I've bought it all there at one time or another. Cheapest place in Seattle and if you haggle even cheaper! Stock does vary a lot!
Another good place is:
Pacific Iron & Metal Co
2230 4th Ave S
Seattle, WA 98134
The have used metal (scrap) and some new - flat bar, round bar, square tubing. Competitively priced.