I picked Squirtle as my starter pokemon so I got the Blastoiseite. I spent a lot of time bringing down my style to get the venusaurite for $10,000 but I can't get the guy to sell me the Charizardite which is what I really want. Is there a certain wait period before he will sell the 2nd one? I believe i'm going on 5 days now it's been and he still only talks about the stone he already sold me. Anyone have any clue as I have yet to see this asked anywhere?
According to this answer, you must have defeated the Elite Four in order for the Stone Emporium man to sell you the other stone.
I also picked a squirtle, but only bought my stones after I beat the Elite Four. I bought the venasaurite first, then he immediately sold me the charizardite stone afterwards.