Actually it is the flag of St. Piran, the patron saint of
tinners but it has been adopted (unofficially)
as the Cornish National flag as St. Piran has been adopted as the Patron Saint
Legend: St. Piran "rediscovered" tin-smelting (the Romans had
smelted tin in Cornwall, but the methods had since been lost) when his black
hearthstone, which was evidently a slab of tin-bearing ore, had the tin smelted
out of it & rose to the top in the form of a white cross (thus the white cross
on a black ground on the flag).
Where can you obtain one? From Derek & Kathy Fields
at: "The Flag Loft"
51, St Austell Rd
St Blazey Gate
Cornwall, England PL24 2SL