Yesterday I was at a store that sells fireplace equipment and gas/electric heaters etc. They had them. I don't need any so I didn't look at the price. Try a place like that. At least someone there will know what you are talking about
Interstate Brick. They are in Salt Lake City. I also think there is a General Shale store out in that area but I do know that Interstate carries them. Should be cheaper than most of the big box stores too. I work for Summit Brick and Interstate is a competitor of ours but they should have what you need.