Ovens are the tier up from an open fire, they turn mush into paper. Anything that looks like an oven counts as an oven, but that does not include fireplaces or similar which just have an open fire locked inside them. You wont miss an oven when you see it. There's two on The Lady Vengeance, and one in the Driftwood tavern, all in the respective kitchen areas.
There are some ingredient vendors, but don't worry about it before Driftwood, because Fort Joy is rough enough without worrying about pixie dust as well. There's one merchant in the Driftwood square, and two lizards in the tavern basement who all sell crafting ingredients.
