The goal is to create the Great Wall of China out of Titanium - which is to say you need a lot of material.
Transmutation magic exists which can turn anything into a pure element (Titanium in this case). However, the amount of Titanium you get out of the transmutation process greatly depends on the purity of the element that you are starting with.
So the question is, What are some of the easiest to acquire high purity elements that you would transmute? This is purely focused on the collection of material and not the actual construction of anything.
With my limited understanding I believe high silicon content sand or coal might be some good options simply because they are easy to acquire, not necessarily because they are pure. How pure is Iron ore that is smelted, for example? Is there a simple refinement process for any solid element that could be used? 90%+ purity being ideal (No gasses or liquids)
Searching for this information has been harder than I expected so any help is appreciated