The fastest would be to buy maple, yew or magic longs. You can use Zybez forums to find the price and rs wiki to calculate the profit.
If you have some extra money you can try joining the "craft n alch" cc. The way this cc works is you buy green d hides from players, tan them into Green d-leathers and trade them to crafters for green d'hide bodies. You usually even make profit this way, but it takes some time to tan the hides.
I don't recommend other things, as they're pretty hard to buy in large quantities.