A useful site for us europeans looking for heater cores was
Nissens. You have to register and log in to search for heatercores, but it's worth the hassle.
Click on
Catalogue in the top menu. Click on
new login to create your account.
Once that's done go back to the Catalogue page and log in. Click on
Radiators. Click on
Heaters.
Select make and model and a list should appear at the bottom. Click on the link on the left in the list for a popup listing the details of that core.
[Edit: Thanks pauldenton, fixed the link!]