I would like to make a fifteen foot curtain rod!

I was in a young couple's home last year they had built with huge windows (he's an architect), and they had used galvanized pipe for curtain rods and painter drop cloths for curtains. Below is Bob Vila's web address for the curtain rods. If you do a search for "drop cloths for curtains", you'll find lots of samples.

https://www.bobvila.com/articles/diy-pipe-curtain-rod/

What type of curtains (fabric, weight, thickness) are you planning to hang up on it, and will the curtains open and close, or stay always closed? What is the maximum width of the top of the curtains? Also, a picture of where you want to mount the rod would be helpful to understand your project.

WOW! this is what I call a party shower! interesting, is this for inside the house or outside? I know that anyone answering this will tell you that you will HAVE to have supports along the way. depending on WHAT you are hanging from it, the supports will have to be 4' to 6' apart. would suggest, if the fabric is light, that the supports be at 5' and 10' from the ceiling.

Buy electric conduit at the width you want. You cancut it to size and also paint it with Rustoleum.

I used a closet rod. Stain or paint it whatever color you like and buy wooden ends for it.

I would go with metal electrical conduit like Dee suggests. I have used them in the past. You will need to support along the length no matter what you use.