With a radiator leak, a small one eventually leads to a bigger one, and sooner rather than later you'll have to remove it to get it fixed.
Back in my motor racing days, a quick fix that always worked was to just crack an egg into the filler. The coolant has to be at normal operating temp for this to work. Pepper was another quick fix, but usually for smaller leaks.
Not sure if Bar's Leaks is still on the market, but that can be used in the interim.
There is nothing (that I'm aware of) for a long term fix.