Or MicroFasteners, or if you live in a metropolitan area that has any semblance of high-tech, your local bolt store.
Note that you probably need more lengths than 1/2 inch, and if the engine is small, large, or old enough you may need more threads than just 4-40. (Cox 049 reedies, for instance, are put together with 2-56 screws with 5-40 prop screws).