I used some heavyweight drafting paper that I had to do the final version of this one.  It's nice paper, but I think the pad that I have is about 15-20 years old.  It is much thicker than most tracing paper, absorbs the ink very nicely.

With computer drafting, I'm not even sure if they make it any longer.  Might be like carbon paper and typewriter ribbon:  a thing of the past (or just about past).