The absolutely best source I have ever found for square nuts is McMaster Carr (out of Chicago)
www.mcmaster.com
They are currently listing sizes from 6-32 up to 3/8-16 (flat top & flat bottom) in plain steel, black oxide coated and zinc plated steel.
If you want flat bottom and rounded top, they list from 6-32 up to 1&1/2-6 in 18-8ss, grade 5 (zinc plated), grade 2 & hot dipped galvanized.
Absolutely fantastic shipping and pricing is more than reasonable.
Only problem I have ever had with them (based on past experience) is you'll end up ordering an absolute pile of other stuff once you get a hold of the (3664 page) catalogue !
Best advice is to have the stuff delivered to your work (or on a day the wife's not going to be home !)......