The Arb has recently made its database available online & free and where you can "...discover sources in 913 North American nurseries for 100456 plants, find 345955 citations to 102,000 plants in science and garden literature, link to selected websites for images and regional information about 6000 plants, and access information on 2302 North American seed and nursery firms."

Here is a link that might be useful: It's slick--check it out here