On the east coast, there is Galbraith Laboratories and Atlantic Microlabs. I have dealt with both, and was always happy with either, but I liked the service at Atlantic better.
However, if you can find a closer place, university lab, or local company, that might be cheaper both in fees and shipping. There are a surprising number of small companies that do their own analyses, and some will do outside ones for a reasonable cost to help pay for the equipment.