The internet is a great way to research and order plants! Here are some of my favorite gardening websites:
I use these plant libraries to look up more information about a plant I'm interested in buying: http://monrovia.com/ or http://davesgarden.com/guides/pf/ I also Google the plant name and read a few descriptions/reviews before ordering something new.
I love this catalog for its gorgeous pictures and great plant descriptions, though it's not the most inexpensive plant source: http://www.whiteflowerfarm.com/
This catalog has a nice selection and moderate prices: http://www.jacksonandperkins.com/
I fell in love with David Austin's english roses, and here's the company's website: http://davidaustinroses.com/american/ My favorites from the website: Eglantyne, Crocus Rose, Claire Austin (only available in the UK so far) and Munstead Wood (only available in the UK).
This nursery is introducing many vigorous new siberian iris hybrids: http://www.jpwflowers.com/ I'm especially interested in the various blue-flowered plants. When I ordered from them, they included an extra plant for me to sample - what a nice surprise!
I've been drooling over the delphinium pictures from this New Zealand nursery for a couple of years and finally ordered some seed this year: http://www.delphinium.co.nz/ Along with the packet of 50 seeds that I ordered, they included another packet of mixed delphinium seed for me to try. Gotta love these small specialty nurseries - I've never had a large nursery include a surprise like that.