Originally posted by dmonroeschool:

if i remember right they don't grow wildly. you find the onion seeds in chest found in the ruined towers and houses. ive been finding tons on my seed.

The problem was not where to find them but that in chest did not have any seeds.
On Moder's mountain i found 6 chhests but they had only obsidian, aarrows and/or saleble items and not every structure had chest. Finaly found them on moutain that i previously visited but not compleatly explored but only one chest on around 8th try.
So basycly on 14 chests only 1 had any seeds