Welcome to All Things Plants
@Edwin !
I can't recommend any specific varieties for you (being at the northern extreme of the US), but I believe you will have much better luck getting large onions from plants, either purchased or grown from seed yourself. If you decide to grow them yourself, I'd recommend starting them in flats or 6-packs or something, rather than directly in the ground, because they are very tiny when they first come up and can be easily overwhelmed by even small weeds. When the little plants get to around 6 inches tall, trim them back to about half of that and then let them keep growing; you will probably want to do that a couple of times, until they look big enough to handle and plant. Once they're planted outside, be sure to keep them weeded, and also make sure they have adequate water; onions are shallow rooted and don't do well with a lot of competition from weeds. And don't crowd them together; there should be a minimum of 4 inches between plants, if not 6 (although you can plant them closer initially and then thin and use the thinnings for green onions).
Johnny's Selected Seeds has a good variety of onion seeds and also has excellent information as to the suitable latitude range for each type.
Hope that's somewhat helpful -- I also agree with the advice to check with your County Extension Office
