You're only looking at perhaps .1A for a pump like this. Your linked Amazon picture doesn't show scale well but the product dimensions show the motor as being only a little over 1cm in thickness.
Go on the safe side and assume it's rated for .050A. Use a 220 ohm resistor (1/2 watt rated) and see how well it works. If you don't have 1/2W resistors then use two 470 ohm 1/4W resistors in parallel. If you feel the motor getting hot then you need to use higher value resistors.