Is there a common source of thallium?
"found mostly in potassium-bearing ores" from Wikipedia- shortly after it said it wasn't found "naturally occurring", by which I assume not as an elemental metal, but as salts or oxides- but then what low-valence metal does?
So, as Bockriss used potassium carbonate (among other things), we can assume if the alchemists used it rather than just potash, they probably didn't have a very pure supply- maybe Th WAS in some of their recipes.