you can make one out of the cap of a soda bottle. Just take the cap off, make a very small hole (like pin sized) in it and screw it back on tight. Then you upturn the bottle so the cap is at the bottom and you can attach some sort of hose over the cap to take the drips to where they need to go. It's best to test the amount of time this lasts for before you go (a larger bottle/small hole might be needed).

PS. If you dont want all the water to evaporate, best to dig the hose into the soil a little bit (like 1/4 inch) so that the water goes straight into the soil and not through the air.