Replacement bleed valve nipples

[bhd]

My hydronic heating system has 30y.o. Thorn panels with brass 3/16" nipples, which have a square drive end and conical sealing end. Over time and by excessive closing force, the cones have developed ridges and may weep. I wish to replace 20 such nipples, but cannot locate a supplier. Can anyone help, please. Bruce
15/4/09 No replies! Are these nipples unavailable in the UK? What happens to those whose nipples have ceased sealing the bleed or may have lost one? Please would someone offer a suggestion. Bruce