I'd try Joe Beatham at Gunshop East Barnet who has a good selection of reloading dies in stock, in RCBS, Hornady and Lee brands. Well worth a call.
Where are you located as those local to you may also be able to advise. In fact many traditional gun shops often will have the odd reloading die set THAT IS OLD STOCK that they are happy to see gone!
Another source might be Gwynedd Firearms up near Bangor and they can't be too far from you. An EXCELLENT shop!