Hi Tricky
My local motor factor had no problem finding a couple of exhaust valves for my Pinto engined Transit
£8'ish each if memory serves me right
Personaly I would inspect each valve & judge it on its own merits.
Once you take the plunge & buy a whole set of valves, you may then find out that the guides need replacing, while you are at it you may just as well have hardened exhaust valve seats ( If your not lucky enough to have an unleaded head )
Before you know it, a load of money will be spent
Mind you, If you are going the fast road cam route, a full re-work of the head might not be such a bad thing