Are you based in the UK?
.303 Mark VI ammo is not rare or even scarce in UK ammo collector circles, and it would not be hard to find a dozen or so rounds, but as others have said, the real hard part is getting it to you!
The real rare .303 ball round is the Mark III hollow nose, so if you find one headstamped "R^L C III" please let me know at once!
Cheers
TonyE