I think they are basically a carriage bolt............If you have an Ace Hardware store near by I'd certainly take a look there. I've had good luck with Ace for many years in my hometown. ...or do you have an "old" hardware store that has been in business for a while.....and you may not find the length you want but shortening them is easy job.
I purchase a lot form McMaster. Always fast delivery and a very easy website to order. I'm guessing on the size, but try this https://www.mcmaster.com/92356A581/ That is listed as a stainless bolt in case you want that...
Also they sell the clutch head driver bits to remove the interior screws should you need that tool...
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