How to write a letter to manufacture asking them to be thire supplier, I have to write a letter to a manufacture asking them to be their supplier for UPVC can anyone help me please ?

You get the name of the person in charge of procurement of manufacturing supplies, and send him or her a package including lists of materials, prices and terms. You could offer an introductory deal good for x amount of time.
You can certainly add a sentence about promises of quality and timely response, but the bottom line is price. You wait a few weeks and then follow up with a short note or postcard about your eagerness to be their supplier. You could offer to take the person in charge out to lunch to discuss it.