Also try manufacturers of lubrication systems for machines.
Several of my machine tools have 1/8" steel lines.
I've bought parts for them from Bijur (the manufacturer), and just called to check on steel line for you.. no luck, they do have copper and brass. I used brass on the last machine I fixed up.

Another place/brand to try would be
http://www.lincoln-orsco.com/
In this PDF it sounds like they do offer 1/8" steel line
http://www.lincolnindustrial.com/Catalogs/Uploaded/Catalog_83/ORSCO_170.pdf

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-