Tilebarn is a great nursery to be sure, but I have known them for around 20 years (well over half my life) and they have never had C. parviflorum available despite repeated asking.
I have 2 plants, however, one from paul Christian (very small....) and another from Ashwood nurseries, Kingswinford, W.Midlands - the latter offer this occasionally.
Final suggestion is use the online RHS plant finder - I seem to recall there are another one or two places I noticed on there (when I looked a few years ago) that do it