Koh Phangan has many shops where you can buy art supplies and crafts. A small shop on Walking Street in Thong Sala, also known as the School Shop for stationery, paints, pencils, paper, canvas and crafts.
There is also a 20 baht shop where you can find useful ingredients. If you're looking for something more specific, you can order it online and it will be easily delivered from Bangkok to the island and won't take too long.
Local craftsmen such as carpenters can make canvas frames. There are also material stores where you can buy fabric by the meter for making clothes.