Iris,
Stones for baking bread, small ones, 12" diameter, are sold in loads of
places in the UK, any department store with a kitchen dept will sell them.
You don't want to pay shipping costs from USA, it would cost more than
the stone, they are heavy.
If you Google "pizza stone" or "baking stone" and limit the search
to .co.uk you should be able to find a stone under 10 GBP. If you pay more
than 10 GBP you're being ripped off.
Don't buy a HearthKit "oven insert", it is grossly overpriced and no more
effective than a cheap stone.
Love
John