As a warrior I only put 1 point into intelligence, relying on intelligence and mana bonuses on equipment for what little magic I used.
Scorpions can be bought from Tingalf in Ars Magicana or (in games started after installing the 1.34 patch) Finnigan in the Ducal Inn in Verdistis. Sometimes Blake in Ars Magicana and Mpenzak in Verdistis have them as well; otherwise they can be found in random drops or some NPC inventories. One scorpion should be enough to take out the dragon rider.
