Protomold[1] are probably that niche, although I'd treat it more as proof of concept than as scalable as proper production molds. If you've validated your prototype that way, you'll probably end up with DFM[2] changes you want to make before dropping 10's of K (or more) on full production stuff.

[1] http://www.protolabs.com/protomold

[2] https://en.wikipedia.org/wiki/Design_for_manufacturability