I would be curious to know what "cornbread mix" Paula was using. Some "cooking books" don't allow brand names .... some recipe writers just "assume" everyone will know what they are using. In this case - if I had to guess ... I would try Martha White yellow cornbread mix.
As for the onions ... if you can't find Vidalia's (only grown in or around Vidalia, Georgia) - try Texas SpringSweet or Texas 1015 SuperSweet - or even Jumbo 1015's (the Jumbo's are really great for onion rings). These onions are low in sulphur and thus taste sweeter. But, even if you can't find any of these sweet beauties, small yellow onions will work in a pinch.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain