There are quite a few carpet knee kickers, carpet stretchers available in the markets. Whichever you are choosing, there are three main types/customer preferentials which are enough for the carpet fitting.
*carpet knee kicker standard: this is non extenable on pin and grip, it is relatively more stable than other extenable and generally the cheapest among the three
*carpet knee kicker extenable with round grip: this can be extended, but with round grip
*carpet knee kicker extenable with square grip: this can be extended, but with square grip