Mogami

Check with Audiogon member imjerrys.
He has some ads posted here, he may be able to do custom lengths.

You also might want to consider moving your amp near the speakers and getting long interconnects to your preamp if that's possible. Long interconnects and shorter speaker cables often work better , even more so if you've got balanced cables.

FWIW, I've been happy with long runs of Mogami interconnects. Many of your recordings are made using them. Cheers,
Spencer

Redco . Have purchased several times from them . 
http://www.redco.com/Custom-Cable.html

Does Mogami make a speaker cable? Anyway, Event Horizon makes custom cables and they use Mogami. I bought an interconnect and coax from them...nice people.

https://eventhorizon-srv.com/ecommerce/os/catalog/?cPath=21

Signal Cable may be able to help you with a custom length speaker cable. They make high quality cables at very decent prices.
http://signalcable.com/index.html

I hope they do as I have ordered some. I ordered them from Redco. I have a strange location which makes speaker wire better.