I used to buy all my seeds from the Organic Gardening Catalogue, before it was taken over, but then they started selling mostly seeds sourced from Europe (because they are organic), so I stopped buying, as conditions in much of Europe do not match the ones I have here. I too love all the small companies which grow their own seeds, such as Beans and Herbs, Real Seeds and Incredible vegetables. Over the years I have found certain varieties do better here than others so I have to source those where I can, which often means Mr Fothergills or DT Brown.
I found the World Seeds site when I was tracking down a particular lettuce which did well in my particular microclimate, but when I saw all those tomatoes I thought I would share in case it was just what someone else needed. It looks as if they source many of their seeds from N America.
I have not had success with saving my own seeds because of disease and poor growth - I would rather start the year with healthy seeds, tubers and plants. We are in a fairly extreme area for growing veg successfully; even down in the valley conditions are better, but I love living up here so I do the best I can in the situation I find.