You can get them from Fisheries supply.
Bilge Pump Switch - Wema-System | Fisheries Supply
This from their website
OVERVIEW
Wema heavy duty bilge pump switch incorporates 2 float activated reed switches protected from damage and jamming by a stainless steel housing. The reed switches are glass encapsulated, and all electronics are fully sealed within the switch head. The system incorporates a 25 amp relay, and the switch is rated for 20 amp pumps. Water in the bilge causes the magnetic float in the chamber to rise, activating the pump when it nears the top, and shutting the pump off as the water level falls and the float reaches the bottom.
• Easy to assemble
• Easy to maintain
• Wire harness is designed for automatic functioning with a manual override