I find a few lying on the ground after raids, so I'd guess some factions use them as melee weapons. Encouraging a tribal faction to attack you might yield a supply of them.

i figured out that playing on tropical rainforest bio gives elephants.

Set up camp mod, go on a hunting caravan. Drown in animal resources. [possibly get raided]

On elephants, mostly. Or go trading for them with a caravan.