A good silicone impregnator is difficult to find at the local home improvement store.
I have always used HMK which is pretty much the leader in marble care products. Their sealers have color enhancing additives that bring out the natural colors of the stone more vividly.
Home Depot does carry a mineral spirit based silicone sealer but I lost my Pro catalog and they probably would'nt understand anyways if you were to ask for the product. My best advice would be for you to call some local marble & stone companies and see if they carry it. I'm sure you will find a dealer pretty easy if you assert yourself.