if you google (camellia japonica 'purple dawn') you will get several hits for nurseries by mailorder. see the link below.
i have 'rose dawn' and found it at a local nursery in auburn years ago but it is probably more common in the trade than the purple-flowered one.

Here is a link that might be useful: search results

'Purple Dawn' (large, flat, imbricated, dark rosy red flower) as been sold as 'Mathotiana' (in the US). Its real (first) name was 'Julia Drayton'. You should find it quite easily under those names.

There is also 'Purple Gown', also known as 'Zipao', which is a huge purple paeoniform reticulata.

And at last there is an other "false Mathotiana", 'Grand Sultan', with looks mostly like 'Julia Drayton' (erect bush with a large flat and imbricated flower), but with a much darker red color.

You can see some of them here :
http://www.lovcam.org/