The place to buy this spell is between the 2 guilds in Selbina. There's 2 mages there. the one to the left sells white magic and the one to the right sells black magic. This is a good place to buy all your spells(other then Aspir, Drain, etc) from level 16(stonega) and up.
Blizzaga does around 215 damage, 280MB. You can get it in Selbina. The only problem with this spell beside's AoE is it makes Fire II look like a puppy and when you get Thundaga Fire II is just sad.