Pretty much, you just find things with large loot packs, and kill lots of them.
If you want something that is AoS or earlier equipment, make the things killed be in Trammel, Fel or Ilshenar, so as to not have the item pool diluted with elf, samurai or gargoyle junk.
If an SE item, kill things in Tokuno
If an Elf item, kill things in ML dungeons
If it's a gargoyle-only item, farm in Ter Mur or the Abyss.
Those are the only real guidelines.
PS: No matter if an item is
a. 90% physical, 10% one element
b. 20% each in all elements
c. 100% in one element
d. Anything in between these
Elemental damage is considered a "Toggle" property. It's either there, or not there. Back when elemental damage was counted in Imbuing for unravel/crafting costs, and runic crafting (or being enhanced with anything), ANY form of elemental damage was 100% intensity property, even if only 10% wasn't physical.