Depending on the size of the hole, the exact location, and the material of the radiator, this may be repairable without replacing the radiotor.
The correct way, it it is on a metal part is to have it soldered. Your garden variety soldering iron and solder probably won't be up to it, tho. You need a welding iron and some pretty heavy gauge solder. But, when I stil lived with my folks (many, many moons ago) I have done it. My Dad has/had one of them puppies. Saved us a lot of money in the long run
If it'* a plastic part, and still a small hole, it can be epoxied. Again, you're probably not going to get away with buying your parts at the local WalMart. But, with the appropriate high temp expoxy, it can be repaired.
The cheap, non recommended way to seal a pinhole in either medium is good old JB Weld. Clean the surface like nothing you've ever cleaned before. Scuff the surface, and apply the JB Weld in thin layers. Get a little bigger each time, making sure the first layer is really rammed into the hole. I actuall fixed a cracked head on a Corsica using the JB Weld method.
Finally, if the hole is more than just a pinhole, you can again used JB Weld or another high temp sealant, but you will want to liberally apply it to the threads of a very small machine screw, which you then screw into the hole. Then, apply the chosen sealant liberally arond the joint between the screw head and the radiator. Then, one this layer has dried, 2 or 4 more thin layers over the whole thing to ensure it is sealed. This method is NOT recommended as a permanent fix. Nor is it recommended for an area where the screw will reduce coolant flow. But, I know for a fact that it does work Not all of us have always had the money for the "recommended" fixes, and have learned how to make due with what we had around at the time...