One note to add: The stuff you'll find in the plumbing aisle at a store like Lowes or Home Depot is probably 95/5 "lead free" (95%tin, 5% antimony) or one of the other lead free alloys. You can try to use it, but lead free solder doesn't really work well for radiator repair and is generally a pain in the *** to work with.
If you want something MUCH easier to work with, 40/60 is also called "radiator solder" and works well for most radiator repairs. 40/60 is 60% lead, so obviously avoid breathing fumes, eating it, licking the roll, etc. and wash your hands after working with it. If you can't get 40/60 then 50/50 "general purpose" or 60/40 will also work (tin/lead alloy). Use acid core or use solid with a paste flux, either works fine. It's important to get the surface bright n' shiny clean first, so also get some emery cloth and/or a stainless steel scratch brush.
You can still buy lead alloy solder but you have to shop around a bit to find it. Not as common anymore because it's illegal for use on anything that has to do with drinking water. Ask an old plumber for some, or you can still get it new from McMaster Carr, Ebay, or a stained glass supply. Just sayin' - don't expect to find it at a plumbing supply.