I've bought from speedy metals before and was pleased but I'm not sure if they carry those sizes of plate. I'm located near Appleton, WI. I think my work gets most of its sheet & plate from our local Alro Steel supplier or Liebovich Steel. I can check with the purchasing agent to verify where we usually get our 1/4" mild steel plate next week.
There is a place up here that deals in smaller quantities that i have heard of people gaving good experiences with, Appleton Steel - Appleton, Wisconsin
Alro Steel, Wauwatosa WI: Alro Specialty Metals - Milwaukee, WI
Speedy Metals, New Berlin WI: Contacting Us
Lievovich Steel, Rockford IL: Liebovich Steel & Aluminum Company