Exhaust manifold bolt size?

One of the two main exhaust manifold nuts has gone missing. When I say manifold nuts im referring to the one that bolts onto the head near cyl 1 and the other at the back at cyl 6. The ones that bolt onto the threads sticking out of the head. So currently I have just one on the cyl 1 side. Id like to go to home cheapo and grab a nut without taking the other off my truck so If anyone knows the specs on it that would be great so I can just go pick one off the shelf party1:

The nuts are 3/8-24.

Lowes may have these with free spinning washer nuts similar to the OEM nuts, otherwise use a washer under the nut.

Ideally you should use a Bellville washer.