Here's a few sites that I visit for information, supplies, seeds and
rooted cuttings.
Lot's of good stuff on this site.  Greenhouse, containers, parts,
ground cover, shade cloth and more.  They also have reasonable
shipping.
Good source of all types of seeds from trees, pine, palms, flowers and
woody shrubs.  I have good success with the germination rate from the
seeds that I've ordered.  Make sure you follow pretreatment requirements.
Supplier of tree pots...the deep kind for trees.  
The above web page is for the US Dept. of Agriculture plant database.  Lots of good information.
Good supplier for greenhouse
supplies, containers, sheet pots, etc.
I get my Hormodin #2 and #3 from this
company.
Novosel Enterprises.  A supplier of all
kinds of pots and gardening supplies
for the hobbyist and professional.
Supplier of all kinds of things from greenhouses,
pots, shade cloth, tools and more.
I've used this site for ordering small quantities of seeds.  
They have excellent pretreatment instructions with each
order. They also ship seeds fairly fast.
Supplier of all sorts of seeds
Bridge Street Nursery
Half way between Walterville and Leaburg.
Springfield, Oregon
This site has tons of useful information for those
wanting to grow a few plants or start a professional
nursery.
George Mander has a nice web site for growing roses.  Lots of useful and interesting information.  Click here to
go there.
Paul Barden has a great site for and about roses. Click here to go there.
Updated 1-21-10
Helpmefind is a great site for rose, clematis and peonies information.  Click here to go there.