Mother Earth News has a fantastic new search engine.
This is the description, copied from the page:
Are you looking for a particular vegetable, herb or flower variety and it's not available in your favorite catalogs? Our new Mother Earth News Seed and Plant Finder lets you quickly search the online catalogs of more than 500 mail order seed companies. All you have to do is type in the variety you're looking for in the search box below, and you'll get a list of links to the companies offering the variety. (For variety names with two or more words, put the entire name inside quotation marks for the best search results.) You can also search for any crop and browse the newest or rarest selections.