Google it for on line suppliers or contact your garden centres.

Camellia sinensis is not of any real ornamental value, so very few nurseries grow it and you certainly won't find it in any garden centres. The few British nurseries that can supply it can be found using the RHS online Plantfinder, but unless you're looking to grow it as a edible curiosity I would go for one of the more showy Camellia species instead.