You can purchase an anti-leak sealant for cooling systems at any auto shop for about 5-10$. it usually stops 80% of minor leaks.
Add DIKE in small amounts until the leak stops, usually about a week apart.
It is also best to drain and refill the car system with DISTILLED WATER to start pure and clean.
When the leak gets stopped, then remove a couple gallons and add the anti freeze.
In the meantime, start saving for the major repair that will be needed in less than 18 months.
My '84 Custom Cruiser had two small coolant leaks (radiator & heater core). I put 2 vials of 'ALUMASEAL' into the radiator. It fixed the leaks in ten minutes. Cost: $4. I had no concerns because it is a 20-year old car. When you have a newer vehicle (with more plastic parts), use 'BAR's LEAKS'.
The first thing to do is isolate the problem. Is it coming from the radiator, one of the radiator hoses, one of the heater hoses, the engine, or out from under the dashboard? Once you have answered those questions, you can move to the next step. If it's a hose, replace it (now would be a good time to replace all of them). If it's coming from the radiator, take it out and have it fixed. If it's coming from the engine, pour in some Bars Leak and sell the car. If it's coming from under the dash and your carpet smells like antifreeze, your heater core is shot. This one can be really easy to fix, or really difficult, depends on the car.
IMO, NEVER put a sealant of any kind into a car's radiator or one to fix a oil leak either.The end result will be having to buy another radiator and or water pump because the sealant gums up the shaft that drives the fan inside the pump.Ask yourself,if it plugs a leak,then it's going to plug up everything else. Assuming it's the radiator you'll need a few thing's to do it this way.Either a rubber plug or rags to seal off where the cap goes because you're going to need to pressurize the system.Drain it(taking it out would be alot easier after draining to do this)and fill it with plain water leaving a little room at the very top.Pump in air under pressure(approx 15lbs) and even Stevie Wonder will see where it's leaking.
The best thing is to isolate the problem and fix it. If your lucky it may just be a hose. Avoid stopleaks - they can gum up your radiator and especially your heater core.