Looks like I need to find a dealer to order new stones -- Woodstock lets you order them online (prices seem pretty reasonable) But Hearthstone makes you go to a local dealer to order parts. I take it the stones are cemented together? I may just try parging the inside of the stone with stove cement and see how that holds up.
I believe their just cemented... I've heard hearthstone has pretty good tech support maybe you should try to contact them before you tear it down. Wouldn't hurt to do a burn and smoke test the cracks to be sure there's a leak.