I am looking for a place in bangkok that sells curtains and more important the railing system whith those tiny wheels that run in a small gutter . The shops outside bangkok i have found so far all sell the wooden poles with wooden rings and that doesn't work very well with large heavy curtains.

Try MBK, there is a curtain shop on the floor with all of the furniture stores. There is also a curtain shop within the MBK Index.

Curtains by Lakkana has what you need. The shop is on the corner of Sukhumvit 71 and Pridibanomyong [soi] 15 Prakanong. Her English is okay if your talking about curtains, no double pricing, and if your looking for fabric she will brew a great coffee free of charge.