The Jaebo mentioned earlier would need to be ripped apart to work, although the cost is low, the effort may not be worth it. However, there are 'expansion' models for the Jaebo unit which connects using an RJ11 (I think), you MIGHT be able to hack it to work with the pi.
I will note, the Jaebos are considered highly unreliable. However, nobody is really sure if it is the pump getting stuck on, the motor controller (for the pump) or the dosing controller itself.
If you just want a selection of pumps, AliExpress has them starting at $3 each...
That said, if you just buy the pump itself, you still need a motor driver for it to interface with the pi. I'm not sure if a simple 12v relay will do the trick or not. I would think something like this would work though:
https://www.sparkfun.com/products/14451