MSC has it. I get the square stuff for chasing punches.
I'd look it up on their website, but it appears to be down for
maintenance at this particular instant.
They're calling it 'square drawn' tool steel or something. Page 1735
of the catalog, if I remember correctly. a 1/4"x3 foot stick of W-1
I had the entire thing listed in my reply to the recent thread about
steels for stamps.