The Dahlia can *not* be bought at the M&P Market in Jeuno like the rest of the flowers. It can only be obtained from Goobbue or found being sold at the AH under Materials-Alchemy.
If you are starting and do not have access to Jeuno just yet; and you want the exit upgrades go to Materials --> Alchemy in the auction house, lol. I dont believe they use a Dahlia for those though lol.
You can also buy it at M&P's Market at the entrance to Upper Jeuno from Batallia Downs. I think it's like 100-200 gil I'm not sure... I've bought an Amaryllis there for the Bastok Exit Upgrade.