Any good hardware store should have a ... hardware section! But you can find a 'fastener' store.
I have whatever the local store carried available qty 1. Yeah, it's the wrong type as it's all rusty. But it shouldn't have any real force on it, I don't think.
(Then again I'm one of the many ppl whose 1up loosens on rough roads. But it did that in 1 1/4 mode too.)
pctloper: it's a socket *head* screw. not a bolt head (or hex head). you use an allen key with it. see photo in amazon link above.
neat little set of charts. looks like they also sell in quantity 1 so if you need a few things, order here or somewhere similar?
https://www.boltdepot.com/fastener-information/Default.aspx