Where can I get a custom piece of jewelry made?

I have an idea of a ring I'd like to have. I want a white gold band eternity ring with alternating diamonds and sapphires...just medium sized, nothing too huge...I havent been able to find anything I like in the stores, so I was thinking of having them made (I'd be buying 3 of them.) WHERE would a good reputable place for me to go? I live in a large metropolitan area, so we have most chains here....is there on you'd recommend who offers this kind of service?