Can I buy willow cuttings from a gardener and grow them for use on sukkot?

There are several different kinds of willow, most (like the common "Weeping Willow"), are not kosher for use on Sukkot. It's best to take a branch from a kosher arava which you bought with rabbinic supervision, and simply place it in water, until roots start growing, and then plant it, making sure to water it well. They are extremely easy to grow. [BTW, a friendly tip: don't plant it too close to sewage or the house, for her roots are very strong and often cause damage or clogging].