Aside from price, why not custome? It really seems like your best bet. Most places are more than wililng to cut a deal these days.
you never know what you will find!

I don't know what you want, but many carpet places sell remnants. You might be able to find one that works and get it bound or serged.

I've been looking for oversized runners for my lower level hallways. Kalaty makes some in 4x10 and 4x12. Rugsabound was having a Kalaty sale. I think it's still going on.

Another idea... MIL 1.0 used a collection of small vintage Oriental rugs for an awkward space like this and it looked pretty cool. She didn't try to make a runner by putting them tightly end-to-end and side-by-side but left several inches between each. All were the same general colorway (dark red ground with the designs mostly in black and white/cream) for some level of cohesion but the patterns varied, some were a little bigger or smaller, some had fringe and others didn't, some were more worn than others, etc. etc. Success would of course depend on the overall decorating style - hers was rather eclectic and heavily based on antiques accumulated over decades.

Two more options. Get an 8 by 10 without a border and have it cut in half and edged. You could have two 5 by 8 runners (or narrower/shorter). Or, use carpet tiles (various companies make them, I am most familiar with FLOR). These would let you cut the rug to any size you want.

If you could use a runner with a middle-eastern or oriental pattern, many 12- to 14-ft runners with a width greater then 3 ft are available on eBay. The prices are often very reasonable.

If you don't mind spending some time, and might like a vintage rug, check out ebay. They don't categorize the runners by length, so it will take a while. But I have seen some great ones on there. In my last house we needed something that was 3' x 18' in muted tones. Needless to say, I moved before I found that rug.

If you are looking for something muted, here are FLOR tiles that could work . .. .

Here is a link that might be useful: FLOR tiles

There is a city in Iran (several, actually) that seem to make a specialty of wide runners, or really narrow rugs, depending on how you look at it. 4x10, 5x13, etc. is not unusual in rugs with the names of Hamedan, Gashgei, (qashkai), and Bahktiar. Maybe their heavily-tribal patterns and colors aren't your cuppa, but I really have found them to be attractive and super-reasonably priced on ebay.
Casey