Where is a good place to buy one and mount on a Reed trailer pump??

http://www.earthquakeindustries.com/minielectricvibrators.htm

Buy yourself two  mount one under the middle top of your screen don't mount it on the top side in case a mixer drive runs his schute  it want get damaged, mount the second one on the rear sloping side of of your hopper, hook them to a double pole switch with the center being off, now where to buy,I have had better luck with VIBCO remember a vibrator is a self destructive devise. plus vibco is made in the USA.

I disagree about mounting it on the bottom of the screen cause concrete will build up on it and it will heat up and burn out because heat will not be able to escape. By mounting it on top of the grate you can clean it and balls and dry loads will go right through the screen. I used to put shower caps on to keep the mud off of it. They are cheap. You can get them at 99c stores.