How heavy a duty?
I have a some 26" long 6mm (I think) black YKK coil zips here - I'll post one down for you to try if you like. They're obviously plastic, but might be good enough? They're closed at the bottom end, and open at the top, but you could easily cut to size and sew across the open end to act as a stopper for the slider.
PM me your address if you're interested.