Bill,
It looks like you hit it on the head.
Forbo is one of the suppliers that I have called in the past. While they are based in Scotland, their Maroleum/Linoleum is made in Germany. They never had the correct thickness. They had 2.5mm : ~1/10"
Restoration Supply has 78" wide sheet of 1/8" linoleum (3.2mm, from Germany - I do not know if it is the Forbo product). That thickness is satisfactory for routing. I am going to see if they will mail me a sample. This product costs $9/sq-ft, as you posted.
Battleship Linoleum has the 2.5mm black or brown linoleum at $6.50/sq-ft
The owner seemed a little skeptical about a 3.5mm product being available.
