Apprentice 3 spells are made by players. You'll have to browse the player marker via your bulletin board or a city broker. The Tellurian spider pet is gotten through the Adept 1 version of that spell. Adept 1's are dropped from creatures, so your best bet of getting one is also buying it from a player.
I'm not sure how the loot/chest system is really done. In my experience, things seem to drop randomly from anything within the items level range, with good things like spells dropping more frequently from group mobs. But I could be wrong :)