Aside from the Black willow ( which really doesn't weep) I did in the thread Cybogo listed above ...
I also recently styled and potted a dwarf weeping willow this very early spring in March for a demo at a garden center program. It is in an interesting modern 3-side cascade pot I made a few years ago and never found a tree for
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Here's a couple pics I took at home right after the demo.
I didn't get any leafed pics later, but, after leafing and some twigging, sold it to another vendor right off the bat while setting up at the Potomac show this '06 spring .
I think the biggest worry to willow bonsai is winter die-back. If you can keep them dormant WITHOUT FREEZING you will have pretty fair succeess. IMO
Dale