There are companies that make cast iron rods and brackets. I didn’t do any internet exploring, though, to see if they make the 144” length, but I think it would be easy to find out.

I would ask why you need a 12 foot run? That is awfully wide. Can't you just put a center bracket?

Perhaps show a photo of your windows

@littlebug zone 5 Missouri

Yes but those are very hard to find in that length it seems. I searched for a couple hours and I couldn't find anything over 120" total length in those.

@Mrs. S Well there will be three brackets, one in the middle and one on each end, so six feet between brackerts. I definitely don't want two separate rods.

Oh, well, in that case, I don't think you need abnormally strong curtain rods. Perhaps just choose rods that seem "sturdier". But 6' between brackets seems to me a quite reasonable and common span. I have several different brands/types of curtain roads spanning about that length, and none have ever bowed.

You may not realize that many (most?) curtain rods sold by the big retailers can be connected end-to-end with a male/male screw that looks something like this:

So you only need 6' rods, but that's assuming you'll have a litttle room for each rod outside the bracket. I like to place one (or two) curtain rings on the rod outside of the end bracket, as then the curtain always covers the side of the window when you pull it shut.

@Mrs. S Thanks for the suggestion, and yes, I do realize that I can patch together two separate rods. As I said getting 144" rods is not the issue, the issue is the bracket strength. These curtains are quite heavy custom made blackout curtains. I won't bore you with the details of their fabric construction but they are HEAVY. I had them on ~10 ft rods in my last home over a 9 foot window opening. I went through several sets of cheaper flat-stock brackets that would constantly slowly bend under the weight.

West Elm for example lists 50 lbs as their weight limit on their 12 foot rods and I'd bet I'm right up against that. In fact I should probably get them out of the box and get an actual weight.

So maybe the answer is to just get two wrought iron sets and patch them as you said.

I guess at this point, you're past my realm of expertise, which is just a diy'er. I can only offer a few suggestions:

1. contact the people who sold the curtains. How do they suggest you hang them?

2. (I know, I know, too late for this part! But with the added expense of super-strong rods, maybe you could think about this: ) Might be better to have blackout blinds, and then decorative curtains.

3. I would check Restoration Hardware, if you're still looking for mainstream retailer.

I had four sliders with 1 rod(s) installed by a pro....Hunter Douglas. Drapery rings were lined with silicone and fabric panels measured to close/cover the slider. Draperies were custom, lined and heavy. No problem. The silicone allowed the rings to glide over the one rod.