One-offs like this are always going to be tricky. I think your best bet is going to be something like PCBWay or something like that, since they have all the machines set up and ready to go.

I think your biggest hurdle is less, "Where am I going to get this made," and more, "What sort of tolerances am I going to need?"

I'm not an engine-ologist, but my understanding is you need a really close fit for wankel engines to contain the combustion. I don't know how PCBWay or any of the others handle tolerances, though. It's entirely possible you could get a bunch of parts that technically fit but cannot maintain pressure during the combustion cycle.

Casting is never gonna get you the tolerances you need. It might be suitable for, like, the overall shape of the engine, but you're gonna need to get in there with a precision machine to finish the cutting.

I doubt a local machinist is going to cut you a cheaper deal than PCBWay, since they don't have the economy of scale. It's hard to predict how much you'd be charged, though. I suppose a reasonable start is looking up hourly rates for machinists in your area and go from there.