If you've got a particular plant in mind, lots of specialist nurseries now take orders off the web or at least let you leave details for mail orders. Just type in the name of the plant. There are quite a few e-garden centres as well like Greenfingers.com.
www.bakker.co.uk now have a web-site for finding & ordering plants for the home & garden. It's well worth a visit with lots of photos etc. You should find something & it's very reasonably priced as well !!
www.gardenersworld.com has got a great selection of plants and products. You can even compare prices on the plants so that you always get the best price. They've got January sales at the moment.