This is by no means a complete list, but it is a list of some suppliers of steel that I have used in the past.
2807 Butterfield Rd.
Yakima, WA 98901
Aldo Bruno (The New Jersey Steel Baron)
Aldo gained fame for his occasional mill runs of high quality 1084. He also has 1095, O1, 52100, 1075, Crucible CPM steels and a host of other steels. In recent times Aldo has gone full time in his steel mongering. Not only can Aldo find you oddball stuff you may need, but he will now be stocking tons of newly melted and rolled steel from a German supplier in a wide range of sizes and compositions! Additionally, for his standard stocked steel material composition certificates will be available for each batch. Now you never have to wonder about specific steel chemistry again! Another great guy to deal with.