Unless things have changed since I last took a car to pieces the starter relay is part of the starter motor.
Small engines (used in generators, riding mowers, etc.) with a starter motor use a starter relay independent of the starter. It's only in newer cars where the relay is part of the starter.
Not just small engines; I know for a fact that on my 79 Bronco (with a 6.6L V8) - the starter relay is separate from the starter (the starter has a Bendix gear instead of a solenoid to kick the pinion in place), mounted on the fender. It was also the same way on my old 1994 Ranger 4-banger. Basically the positive from the battery goes straight to the relay, then from the relay to the starter motor; thick cables all around - lots of current.
Such a relay is different from a "starter solenoid" - a solenoid is something used instead of a Bendix gear system to kick the pinion of the starter motor in and out from the flywheel ring gear (whereas the Bendix uses a spring and the under-run/over-run of the speed of the flywheel vs engine speed vs starter speed to kick in/out).
Anyhow - the starter relay looks something like this:
Ultimately, though, such a relay is intermittent duty only - if you try to keep that much current running through it constantly, it'll melt or catch on fire.
High-current relays for AC mains voltages are typically called "contactors", and are typically used in home air-conditioning systems. Most of the time, these contactors use 24 volts AC to operate the coil (in the air conditioner is a transformer that converts the 220vac to 24vac); even the few 120vac contactors I have seen use 24vac for the coil (but there do exist ones that use lower DC voltages as well).
Another good place to get high-current relays are from/for older electric golf carts (generally rated for 36vdc at a ton of amps - and continuous!).
Something else to keep in mind is that for the most part, the contacts are rated for current, not voltage; you'll often see relays with a contact rating of something like "250vac/20A and 12vdc/20A" - so mainly pay attention to the amp rating of the contacts, not the voltage (I once replaced a relay in a waffle maker with a 12vdc 40A Bosch automotive relay because I couldn't source the proper Osram in time for a restaurant to open - the relay controlled the voltage to the heating elements - it's worked fine for about a year now in daily, constant use - anecdotal, though).