There are factories that specialize in small batches. Just some quick searching seemed to suggest that you could be getting O(1000) parts a day at a cost of a few thousand dollars (for the initial setup) plus a few dollars a part [1]. I'd imagine that the lead time is a few weeks rather than days, but if there's already a stable design wouldn't this be the best bet?

[1]: https://www.wired.com/2013/01/protomold/