There are a lot of nurseries which also do mail order. I've never ordered *bleep* willows online, so here are some of my general favourites.
Crocus.co.uk I've found to have a reasonable selection and always supplies good quality, fairly large plants. On the downside, their prices are comparatively high - I think you're paying for the reliability (and the nice website).
Cotswold Garden Flowers has a very good selection at good prices, run by a proper plantsman - though I did once have a batch of plants arrive which I didn't think were as well wrapped up as they should have been. However, the company has since taken the mail order back in house, so that shouldn't reoccur.
I ordered some plants from Duchy of Cornwall nurseries via mail order and was rather disappointed with the quality of the plants, though actually all the plants settled down quite nicely. I felt that they hadn't picked out the plants to send as carefully as I would have chosen them if I was buying them in store.
