Can you buy a replacement pin if you lose it and if so where from?

While trying to free a stuck/corroded radiator valve pin, on a Westhermfour thermostatic rad valve, I pulled a bit too hard and the pin came out completely. In my shock I dropped it and it fell down the smallest gap round the pipe! I have made a temporary repair but wonder if I can just replace the pin or do I have to buy a complete valve.