Flower threading is an art which I started to learn when I first moved to Phuket in 2000 and unfortunately did not continue to learn.
These types of flower garlands are called 'Uba'.
The local markets are a place where I saw the most flowers being threaded at the same time - Rung Market (behind Robinsons).
It is still a popular skill in Phuket and some are sold on the side of the road and the larger ones (more detailed) are sold in the Flower Shops (Suan Luang for example).
However the very ornate and detailed ones can be purchased in Bangkok.
But the joy of watching the flowers be threaded in the Local market is lovely.