I've just bought a set of
SRB's own brand that turn out to be HiTech
£12.50 each or £46 for a set of four from memory
They recommend Kood adaptors that are about half the price of Cokin.
If you are really set on Cokin, they also stock them.
Everything is on their web site. They are based in Bedfordshire.
The cast issues are apparently due to the Cokins being batch checked, rather than individually checked.