+1 ...... for Kate at Hideout.
However, I have managed a DIY fix for this problem in the past, when SWMBO managed to break the zip runner in two. We went to our local market haberdashery stall guy with the wife's FT jacket (old but treasured) - he managed to find a match for the zip size & style of tooth closure. Sold us the shortest zip he had - £2.50 if I remember rightly. We cut the zip & removed the runner. Then, at the very top of the right zip run (right for ladies, left for men) I had to find a way to work the runner onto the zip. Sometimes there is a staple, sometimes these are oversewn into the jacket - in the end we had to sacrifice the very top tooth of the zip, rather than take the jacket apart & wriggled the runner onto the zip run. took the runner down & connected up the zip. It was all a bit stiff initially, so we used some graphite as lubricant, which soon eased things. Then we over-sewed the missing tooth area at the top, to stop the runner de-railing, which was hidden behind a weather flap & the neck fastening anyway.
Worked a treat, the jacket is still going strong years later. We did the same with a set of HG ladies leather bottoms too, when both zips failed in quick succession - slightly different technique though. Ok, it's a bit of a bodge, & inappropriate if the jacket is nearly new, but an economic repair if the cost of a full zip replacement cannot be justified. Obviously there are safety implications, so the strength of the finished job is vital.