This online library is designed to give data about a myriad of topics so
that teachers can use them for teaching statistics. I think it could be
helpful for designing story problems in general.
While not particularly user-friendly creation-wise, it is pretty easy to pick up on what you can do. It has math topics: integers, fractions, concepts, and geometry. Within each, you can choose specific concepts, level of difficulty, and the number of problems. Then, simply send students to the URL, and they can take a fill-in-the blank quiz and see results immediately.