The store is in the app version, with in-game purchases. The PC version from Big Fish is a onetime purchase of the game and doesn't involve further expenditures.
In that version, you get 10 lava stones from the little hut in the pool of lava, with a regeneration time of 24 hours. Also for completing puzzles and once in awhile found in the cracks. For some odd reason, the yellow statue puzzle paid out 200 stones, while most puzzles are 30.
My mausoleum is filled to the max with boxes, in stacks of six. Yay.