There are a lot of tailors in Thailand which can make you a complete suit in a record time and which will fit perfectly to your body. Some are better than others and in Koh Chang, you can find a few of them.

Tailors can make any kind of clothes, from a simple shirt to a wedding dress. They have a large choice of materials and colours, from cotton to cashmere. Most of them have a lot of catalogs from different collections where you can choose what you would like to wear. But if you already have an idea of what you want, the best is to print a copy of any picture and show it to them, you will save a lot of time.

Normally the process has a few steps. At the first meeting, you will choose the model, the cut, and the material and then the tailor will take your dimensions. They will ask you to come back a few hours later or the following day to try the cloth before it's finished. They can still modify it if you don't like it or if it doesn't fit well. You will have to come again to do the final test and take the cloth with you.
This may sound long but it's pretty fast and it's better to do it in this way to be sure that you will get what you have asked for.

But there are a few things you have to know before going to a tailor shop.
All of them are not very serious and they will try to fool you, pushing you to buy what you don't need. They will get you inside their shop at an attractive price but once inside what they will propose you will be totally different. And of course, they will pretend that they are doing a special discount just for you.
Lots of guides or resorts get a commission if they send customers so don't believe them too much. If you have time visit a few tailors and compare the prices, don't accept the deal immediately.

I can recommend one tailor in Koh Chang where I have been several times and who has never tried to fool me, I give him my full trust and all my friends who have been there had been satisfied by the results and prices. You can step inside their shop and you will not feel the pressure to be obliged to spend your money and the quality is very good for the price. It's Koh Chang Milano Fashion, located White Sand beach and opened every day during the high season. The quietest time of the day to visit them in the morning or afternoon when everybody is at the beach. But the best is to go in the evening after or before your dinner, you will not feel so hot to try clothes even if the shop is air-conditioned.
Check at their website where you can see lots of models they have made for their customers: