Looking for blanks for steel stamps

You can buy tool steel or drill rod from multiple sellers including Online Metals. Both cylindrical and square form are available. You might be able to order a length and hav a local machine shop cut it to length.
I have had good success making gravers, not such good success engraving😎 however.
Some hardware stores, not HD or Lowe’s, carry drill rod an acceptable material for your use.
Hope this helps.

here is a great book an making stamps…