Oddly enough there used to be a company called Evans & Howard Firebrick company in St. Louis and one called Laclede, not that this helps you in any way. If you check places that sell Wood stoves, they should have some or can lead you to somewhere, they should also have refractory cement. You want the soft, light bricks, not the hard, heavy ones. Although the dense ones will work, they will take longer to heat up and are much harder to work with. The softer bricks are made for use in gas kilns and have a better insulation value. Im not sure how soapstone would work but it is supposedly resistant to heat. Might be a little more expensive than firebrick though. There is a chimney place on St. Theresa Ave, you can try there, or Arnold Stove in Arnold.
Or like Stacy said...