While I am not defending Onan, they buy the carbs complete from carburetor manufacturers like Nikki. I can understand that it doesn't make sense to stock every nut and bolt that is bought as a complete unit from another vendor. A lot of automotive parts are the same way.
Since you have a complete carb for $40 bucks, use the bolt you need and then try to part it out on Ebay. If you find it is not worth the time and effort, you may see why Onan does too. When they are selling generator units for 4000 bucks a pop, why waste resources on a $1 bolt?
When I rebuilt my Onan, I learned that OEM Onan parts almost cost their weight in gold. When they do stock a part, they charge through the nose for it.
Glad to hear you got what you needed though. Hope you are able to get her back up and running.
John