[Jack21]

McMaster-Carr (mcmaster.com) has arguably the most extensive line of hardware imaginable. Hit your local True Value hardware, they might have it. (My local True Value, after seeing me several times a week for over a year has offered me a job.) You're in the hardware capital of the world (Chicago). Try flipping through the yellow pages, and making some calls.

That being said, if you can find this in a 6", cut off 1/2", and clean up the threads with a file, and 5/16x18 die. If it's a critical bolt, consider a grade 8 instead of stainless which is Cadmium plated for corrosion resistance, and it's stronger.