Unless you're going to use a food processor, the best way to dice onions is simply learning to use a decent chef's knife. You need one anyhow, as it's hard to get far in cooking without at least the one, onions or no onions.

I'm happy with the modestly priced Mercer knife I got earlier this year. Here's the steel I got with it to keep it honed. I cook regularly but not every day (it's just me, so leftovers are a thing), so hopefully it'll be a while before it needs proper sharpening.

It's plain-jane as far as a knife goes, but it's heavy, super sharp, durable and reliable. It was cheaper when I bought it, though...