I think the Childens Place has slims up to size 14 or 16, with the adjustable waist. Their website is weird, though. You have to pick jeans in the left side navigation, then slims over there, too. (I would think they would just have slim / plus as a sizing option when you're looking at the pants, but they don't)
Old Navy also has slims, and they typically run long in the leg, at least in adult sizes.
Maybe try looking at places like Forever 21 or Charlotte Russe, where they have the super-teeny juniors sizes, like 00?