I checked with my metallurgist brother who has a lot of experience with foundry work, and he recommended a couple places here in the Minneapolis area:
http://www.carleyfoundry.com they do both sand casting and investment casting of aluminum
https://surecastfoundry.com he said they seem to welcome low volume / prototype work
I know Minneapolis is 2,000 miles away so you couldn't just pop over to visit in person, but you could maybe deal with them over the phone.