Here's one of my two SL35 signs I got from the RIDOT yard when i visited them when I was 8 in 2006. Made by 3M. I plan to get some sheet metal for these when I get my own place.
Yep 3M and Avery Dennison are two companies I know of that makes reflective sheeting, normally the signs are made at either a private or a DOT owned sign shop.
Not really sure who makes the metal sign blanks though.
Measure the sheet and contact any metal supplier to get a sheet cut to size. You can order either chromadek or aluminium sheet as a backing. Just remember that that 3M adhesive is really aggressive, and does not _EVER_ let go once it has touched a surface. Another is to contact a signage company and get some acrylic sheet cut to the size.
Who makes the metal signs though? The metal part,
Not really sure who makes the metal sign blanks though.