I have been researching this on and off this spring and summer. I suggest you google keywords willow, living structure, dome, fedge, willow rods, etc. (you'll start to get a feel for the jargon after reading a few sites) and select the images tab for ideas to get you started.
Two websites I have found that sell rods (long cuttings) and even rod kits for domes, etc. are
www.bluestem.ca (click the link to willow uses in the narrative on the homepage) and
www.yorkshirewillow.co.uk. You will find lots of information on these two sites: tutorials on making domes, information on the characteristics of various willow cultivars, and plenty of pictures. I think I found a treasure trove of pictures by looking through the resources of a wikipedia page on fedges or living willow structures...There are lots of other sites and blogs with individual projects, but the research is part of the fun! This practice is more common in the UK, so I've not found any nurseries in the US that provide willow rods for structures, although Bluestem (Canada), above, does ship kits to North America.
As for which variety to use, Salix viminalis seems to be the preferred variety to use, as it tends to grow very long shoots that are strong enough to be suitable for larger structures. One of the two websites above has a nice table that shows bark color, flexibility and length of the rods, and other tips to help select the right variety for your specific project.
I'm going to grab a curly willow I saw today (not ideal for living willow structures) to plant as a donor in the low/wet area by the pond and I hope to start some mini-domes for the ducks next spring. I'm hoping to line them with hardware cloth and grow them around some sort of frame that will allow me to close them at night; I've had too many nesting hens refuse to come out of their secret places at night to get locked up only to lose them to sneaky coyotes

Even if I can't figure out how to secure them, they should provide some shelter/shade and won't be as ugly as hardware cloth partitions everywhere!! If I can keep the mini-domes under control, I may start something more elaborate for ourselves
