I think you're referring to the Pivot Knuckle Bolt (#V below).
The problem with replacing this bolt with hardware store stuff is that you need the bows to pivot smoothly, yet still fit snugly. Because of wind pressure, there's a lot of stress at that connection point, so a loose fit may cause issue.
The cotter pin solution is fine, but might be a short term thing, unless you get a really nice fit.
You can call 800-845-3567 and ask our support team for some. Don't know what we charge, but it's always a little more than it should be (we aren't really in the parts business). Quad may also carry them. They cant' cost that much.
For $25 bucks you can get "quick release bow knuckles" that use a little spring mounted pin. It's a nice upgrade.