The air compressor unloader valve is usually built into the pressure switch assembly, though it's sometimes found on top of the switch. Contact the air compressor manufacturer to buy a new assembly.

Onboard air can ... way check valve (PN VAR-92831), Viair four-way fitting (PN VAR-92855), Viair Stainless Steel braded-leader hose (PN VAR- 92793), and a Viair 110/145 psi pressure switch ...