Parallels are used in machine shop environments to assist in measuring or holding work pieces in drill presses or milling machines. They are typically made of steel machined to tight tolerances (link to amazon version for reference: http://amzn.to/28PGxhU). I've found that my 3d printer holds tolerances well enough to make this a cheap/disposable set for home use.

The bars are 3/16" thick, 4-1/2" long, and vary in width from 3/8 to 1" in 1/8" increments.

These parts don't need to be very strong to function, even so I still printed everything at 30% infill.