Clematis Jouiniana 'Praecox'

I love this. where is best place to buy and when best time to plant please?

It's gorgeous Helen !

I would say the best place to buy it would be your local garden centre.
This way you can actually 'see' what you're paying for, mail order's fine for some things but on these i'd say buy where you can see it.

As for planting time, i would do it whilst the soil's still warm so do it soon or wait until spring.
I'd favour now though, this way it can settle in before the coldest of weathers arrive and, there's going to be no risk of dryness and warm sun like there could be in spring !
They 'hate' drought and planting in spring can mean extra amounts of watering needed !

thanks very much. a trip to the garden centre for me tomorrow then.