I need to buy 2 zippers of length approximately 20 cm (8 in). I can't recall ever seeing a shop that sells zippers.

There are several shops on Chang Moi Rd. One shop does fittings for making bags and also has leather scraps whereas the other is a material and sewing place. Both are on the right with the second shop being on the first intersection at Wararot market.

Cmkiwi is correct and the shop which sells buttons, buckles ... every one ever used by mankind since Adam was a boy ... also makes zippers to length if required size is not in stock, which would be unlikely.

IIRC it's on or just near the intersection with the wrong way road at Chinatown entrance on Changmoi road.

The name is 'Kul shop'... standing in front of the red entrance gate to China town, its about 100 meters to your right.

There are several shops on Chang Moi Rd. One shop does fittings for making bags and also has leather scraps whereas the other is a material and sewing place. Both are on the right with the second shop being on the first intersection at Wararot market.

I found the shop that has materials for making bags today. Bought a long zipper which can be cut into two shorter zippers. So I found exactly what I needed for only 10 Baht.

I have a sore back and getting two small cloth bags made that I will fill with rice and use as a hot pack heated by the microwave. My soreness responds to applied heat very well.