Ever since the growing population of players getting into the Herblore skill, there
has been a high demand for water-filled vials (wfv). You can easily get these vials either by purchasing them from some stores or by making them yourself (easily done by filling up empty vials with water from a water source). You can purchase empty vials from all Herblore stores, and even some wfv from specific General stores. Wfv are worth around 150-300gps each, depending on the buyer.

The best place to obtain wfv is in East Ardougne, at the General store west of the
South Bank. The water-filled vials at the store cost a measely 12gps each, and has a
stocking limit of 150. It barely takes a minute to make a round trip from the store to the South Bank to deposit the vials, so this process may not be too time consuming in its transportation. The best way to profit off of this strategy is to purchase all the vials you can from the store, depositing them along the way, and then switching off onto another world to do the same process. However, because of the vast majority of players looking for an easy profitable source of income, there is a high possibility that only some of the worlds will have a high stock number of wfv at this store. But indeed, you still have your chance to make an easy 50k-100k per hour with this option.