If I can't find anything named "scallions" at the store, what else can I look for?
According to Wikipedia:
Scallions (also known as green onions, spring onions, salad onions, green shallots, onion sticks, or syboes), are the edible plants of various Allium species, all of which are "onion-like", having hollow green leaves and lacking a fully developed root bulb.
In the grocery stores I've been to most (east cost of US), if they aren't called scallions, they're called green onions. I've never seen grocery stores label them with any of those other names from Wikipedia (though I've seen recipes calling for syboes).
Then of course there are stores / vegetable stands where nothing is labelled, in which case knowing what they look like is the most helpful ;) (Google image search yields plenty of decent results.)