Check with companies like Cole-Parmer or Forestry Supply for a sampler; however you can easily make your own from PVC pipe. Attach two ball valves together, separated by a section of PVC pipe appropriate for the size of sample you need; make sure the ball valve handles are aligned and open / close in the same direction. You want the handle to open towards the bottom of the sampler ( I'll explain why in a second). Next, attach a length of PVC pipe to the upper ball valve suitable for the depth you want to reach. Attach a rope or cable to both ball valve handles, so that they can be closed at the same time. With the valves about 75% open, lower the device to the desired depth, close both valves with the rope, and you now have a sample that contains only water from the zone between the two ball valves.