Hi Morganna - Nice to see you on the forum.
Although it's not on your route, I'm pretty sure you can get onion starts at Red Wagon Plants
in Hinesburg. According to the website, they offer four varieties of onions, as well as scallions and leeks.
However, since it's out of the way, I'd recommend calling first. I'm not sure how quickly they sell out of things once planting starts in earnest.