What we're looking for are available on Amazon if you can't find them locally. They come in metric and imperial sizes, and although they are identical in terms of width and length, the imperial threaded nuts will only fit the imperial threaded bolts, and the same goes for metric. Here are the links for the metric pieces:
And here are the links for the imperial pieces:
The only real difference, besides the threads, is that the imperial bolts have a wider head, which I actually prefer, but there is nothing wrong with the smaller metric bolt heads. I hope this helps.