In addition to the commercial directories, we recommend The Garden Gate, written and maintained by Karen Fletcher. A Source Book is a reference book much loved by gardeners throughout the country. It's organized so you can look up a certain plant type, such as daffodils, and then find mail order references that include daffodils in their offerings. The book includes other resources as well, such as sources for gardening supplies, lists of magazines, books, libraries, and plant societies and professional organizations. Access it at Find a Source.
All content found in the book is included here.
A bonus is that material is updated often so you don't have to wait a year or two for a new printed version. For those who prefer to curl up with hard copy in front of a warm fireplace or sunny deck, the printed version is still available in book stores. If you have an email account you can subscribe to any number of lists and participate in email discussions, get your own questions answered and offer information or advice to others.