Does anyone know where I can find a bolt for a set of Proline cranks that holds the sprocket in place? Can't seem to find anything that matches at Home Depot, Lowe's, etc.

Should be the same as the Dans Comp Poverty crank arms. Give them a try.

Thank you -

I also found a thread on here where someone shows the specs for the classic Flight Crank hardware, does anyone know if that hardware will also fit modern Prolines? I know the spindles aren't interchangeable, but just the bolts and everything.

You can try Ace Hardware