Looting bag

The two typical ways I think are stepping over the wilderness ditch and killing low level monsters which drop them at a high rate or buying them from a slayer master for 10 slayer points.

If you ever find yourself being a whale of a UIM, it's definitely recommended to use slayer points to buy looting bags.

Travel to the Ferox Enclave using the canoe system or games necklace/basic jewelry box teleport. This is a safe area in the wilderness to put tradeable items in your looting bag. The only way to get the items back is to die with the looting bag, which breaks the looting bag, but all of the contained items drop to the floor.