Protomold.com is your best bet, ultimately you can make a larger run off the molds where as milling won't tell you anything about the viability of it as an inje

Protomold.com is your best bet, ultimately you can make a larger run off the molds where as milling won't tell you anything about the viability of it as an injection molded product. Also injection molding is a bit more complicated than just drawing up a shape so you might need some expertise if you are going down that road.