I found one on a paid site that I use. It is about $60 for the cheapest registration and you are able to download up to 5 models from this site. Each model has about 10 available formats to choose from. Some of them will directly import to SolidWorks, others will need a third-party conversion software.
The size and shape of a radiator depends on the application. If you have an idea of what you need, could you just model it using solid blocks without all the internal detail? You could also take some measurements off your car radiator and scale it up or down.