After getting an Empty Bottle you can get as much Lantern Oil and Red Potion for as cheap as 0 Rupees (the unsafe way) or 1 Rupee (the easy and safe way). Go to the small shop near the Forest Temple run by the talking bird. You can take whichever item desired. If you do not wish to pay, run. If you are not fast enough the bird will attack you repeatedly, then when you return he will call you a thief and may attack you again. However, if you want to use the safer method, grab as much as you desire, then walk over to the box besides the bird. Only pay 1 Rupee no matter what the price is. He will make a comment about how cheap you are getting it for, but will not attack.