I don't know where you live in N. Ca. but if you are near Santa Rosa there are two good resources for you check out : Wheeler Zameroni and Johnson's Stone yard.

Both carry a wide range of flat stone other than the commonly seen Arizona pink toned flat stone.

There is also American Soils products in Berkeley but they have such horrible customer service and their prices are usually a few dollars more than anyone in the area.

There are plenty of different brown and grey stones to choose from.
Bouquet canyon and California brown stone are two of the closest stones that mimic N. E. Bluestone.

From there the list is long , from Whisper Creek stone quarry , Three Rivers, Moss Back , Idaho, ect...... .
Lots to choose from if you look in the right place.

Thanks for the info; we live in the East Bay so Santa Rosa is not too far, and good to know about American Soil since my husband is fatally attracted to the granite boulders they show on their home page and wants to buy stuff there! Any good online references for looking at stone photos without driving hither and yon? I have found the usual with google but as a professional perhaps you know of other resources?