How to create a Large Block of Magic Clay
In order to create a Large Block of Magic Clay you'll need to collect Large Blocks of Clay, sold by almost any Pottery Supplies vendor in the game and find an Enchanter with the spell Mass Enchant Clay or Enchant Clay. These two spells are learned prior to level 10 so you don't need to find a very high level Enchanter to Enchant your clay.
Those of you reading this on a TLP server, in particular on a TLP server that has yet to reach The Serpent's Spine expansion pack... It's recommended that you find a higher level Enchanter to do this for you as the med times on your server will be extremely slow and it can take 30 minutes or more depending on how much stuff you have to enchant.
Additionally, it's important to keep in mind that you're asking someone to spend their time doing this for you. Tipping them or donating in some way or another is not only a nice thing to do but the appropriate thing to do.