Well, apart from the Opel Corsa B model radiators, you can also take a look on the Renault Twingo radiators. They're both made by Harrison.
Here are the pics with the measures:
Other than that, you have a lot of variety from older models (Volvo 340; older Audi 100, and some japanese cars - Toyota/Subaru/Mitsubishi) prior to 1984 I guess) but you've got to have some luck to find a radiator that still is in a good condition.