yashiharu:
i just found that the water flow is too fast (or i have to change the whole final work ...)and I got the motor shield, 2A max.
but i have 3-4 pumps to work (unsync or sometime sync), thats why i go for relay (for it's external power)
Option 1.
If you want to set the flow, and leave it, not adjust it programmatically, then you can use power resistors to divide the voltage and throw away the excess V as heat.
Resistors could be discrete, or rheostat. Power calc says P=I*E, 9W. All headroom and that's about 15W. At ebay rheostats are $8-10. With that can vary the voltage/flow easily, or much cheaper with discrete resistors, but trial/error to pick values.
Option 2. (Partial fix, as you have one motor shield.)
You can always use the motor shield to control PWM for any two motors. Just send the power wire already going through a relay through the motor shield to a pump, and control by other pins. It's not a -bad- thing to use -both- relay and PWM control. PWM turns voltage 'essentially off,' but relay completely disconnects it.
By sync do you mean controlling the on/off together, or equalising the flows, or both? PWM can -probably- control flow tighter than rheostats can. And use resistors where sync counts for less, and save a -buncha- $.
The motor shield allows 2A for each motor which is close to the limit but not a real problem. Just make sure there's free air around the board, not too enclosed.