Vertical adjustment slider for VESA mount?
A real cost-effective way to position a slide is to use a screw drive.  These can handle the load, move the payload rapidly and, if geared properly, stay where you put them.

You should be able to make a unit cheaply like so:  

(Click an image to see it full-sized)



This would use:


Two good-quality drawer slides.
Plywood for the moving platform, and optionally a plywood base for the whole thing.
3 feet of "all thread" rod (the larger the diameter, the better).
A matching nut or a threaded block for the drive rod.  If a plain nut is used, then you also need some means to affix it to the moving platform.
A reversible motor and gearing.  Old drills, shredders, electric car-window lifts, etc. are good sources for this.  Or you could buy something new.
This is the part that will require most of your ingenuity, as there is nothing off the shelf that I am aware of. But it's not too hard to rig something up if you've a little fab know-how.  
If the motor was DC, a power supply for it.  Old computer supplies often put out plenty of juice at 12V, if a 12V motor is used.
A suitable switch for the motor, usually DPDT is required.
Misc hardware, paint, axle grease for the drive screw.