Why is this plant not avilable to buy?

It is because it is such a thug that few people would want to buy it and plant it in their garden.
You need to look in the cheapest nastiest type of Garden centre.they often stock it as it is so simple to propagate and grow on. Sure I have seen it in B&Q type places.

In one season it will smother everything in site and ruin the balance of any rockery or bed as I learnt to my cost in an inherited rockery at a house we bought.It is very pretty though,so maybe you have space enough for it.Seems readily available on google.

Added to this, I told another member that seeds are available from T&M. I don't know how easy they are to germinate, though.

Maybe try e-bay?

Or beg, borrow or steal a piece from someone who has already got it

Anyone who's got it (like me, but I live too far away) would happily give you some, even a total stranger. It is pretty but it overruns and even kills other plants. I'm trying to eliminate it from parts of my rockeries and keep it to the huge sandy wastes where other things won't grow.

That made me laugh Chris having read this yesterday in a parish magazine.
[quote]
When I was a new gardener I read that Marjorie
Fish, doyen of post-war cottage gardening, recalls
that when friends came begging ‘with big baskets’
she invariably gave them quantities of a pretty white
rock plant, Cerastium tomentosum and known to the
cottagers as ‘Snow in summer’ assuring them that ‘it would do well’. I thought this was a really good
idea, especially since many plants I’d bought just seemed to have a death wish; so I bought some of
this magic plant and saw that it didn’t just do well, it wanted to take over the whole garden. Perhaps
Marjorie, in the modern parlance, was just ‘having a laugh’; possibly she wanted these gardeners to
take their big baskets elsewhere – who knows? – but the lesson to learn is this – beware of gardeners
who seem to have a superfluity of anything and never, never, allow yourself to accept and plant such offerings.