Melanie - there are no WW 'bonsai' seeds, only seeds of a WW. Once the trees have grown (3-5 yrs or more) someone may choose to 'bonsai' them, but that's all. There's no such thing as bonsai seeds, just seeds of regular trees which someone may train as a bonsai. Google seeds (or try Seedrack.com) and find a company to order from, but you'll be better off to learn something about bonsai first (www.bonsai4me.com, and books) and understand that you'll have a tree to work on a lot faster if you start out with something already a few yrs old (from a local nursery) than from seed - most bonsai are cut back from larger stock, which is why you get those gnarly old looking trunks and little branches (newly grown in fact),etc. PS - WW don't make the best bonsai.. they're a bit fussy to train.

You can take a rather large branch of most willows and root it in water, effectively giving you thick bonsai stock in a very short amount of time. Develop some side branches, hollow out the trunk and make a gnarly old willow in a few years.

I'll echo al dente by saying that you can root pretty large willow cuttings, so I would not go the seed route. Weeping willows can make some delicate beautiful bonsai, but they need to be on the large side to really work. They had a great workshop at Brussell's Rendezvous this year. I saw some really nice 3' tall trees going out of there.