Here are some great resources to check out when you're shopping for plants and seeds by mail:
Mailorder Gardening Association
Includes many tips on shopping, ordering and caring for plants, plus lists of catalogs and news about mail-order companies.
Cyndi's Catalog of Garden Catalogs
This site lists more than 2,000 companies in the US and Canada. They are organized into lists according to their specialties, and many entries include ratings on quality, prices and more.
The Garden Watchdog
This section of the Dave's Garden website is a directory of mail-order companies, with ratings and comments by gardeners across the country who have tried them.
This directory lists plant and seed companies as well as sources for tools and other garden-related things.