If you like rigging up your own stuff, and saving a buck, you can very easily make your own.
Here's just one version found on the interwebs.
Linky
The one I made is mounted to an L shaped board/base from plywood, and you have to put in restrictors, which are just two one inch sections of very narrow tube (toothpick sized inside diameter) spliced inline near each end. I found little nylon spacers that worked as restrictors in those drawers of Hillman fasteners at the hardware store. Most Ace and Our Own, True Value, have those drawers.

The restrictors damp the fluid, so it doesn't "boil" from the intake pulses and also slows the rise and fall of the fluid so you have time to make fine adjustments.
Here's mine:

-K