Are you talking about a relay for the starter circuit? As opposed to the "solenoid" that must sit on the starter motor itself. The term "solenoid" usually refers to the latter. A starter solenoid pulls in the
gear to physically connect the starter motor to the flywheel, at the same time it closes a circuit to apply 12V to the starter motor. It's called a solenoid due to moving the
gear, as opposed to a relay which only closes
electrical contacts.
Starter circuit relays are used to apply
power to the starter solenoid thereby reducing
current requirements of the ignition switch. A proper starter relay is fairly easy to find on-line - look for an automotive 12V relay rated at 100A. Or you can use the same type of relay used in voltage-controlled-relay
battery isolators.