Try out this place:
It's a real old-time hardware store located up the river from Trenton in Lambertville, not too far away.
They also have a steel department with a good selection of stuff. I purchase my steel there and have gotten some 4"x4" steel tubing, so they probably have what you need.
Upon entering the website, click on the "steel" tab, and have a look.
This is about the only place around that I have found that doesn't have a minimum or cater to bulk purchasers.
Hope this helps,