I was looking a few months back as I was considring extending one of mine. Managed to track them down as being remote cables used on boats for remote throttle and steering controls. My local boat chandler was listed as a supplier. Didnt do it in the end, but will try and find the details and post a link on here.
marlin get their cables from Aquafax.co.uk. they are stockists for ultraflex cable. speak to the guy there and he will advise. all lengths available up to 30feet long. there is a heavier duty cable available than the one i was supplied with but it would mean changing the cable end mounts. was tempted to go for thicker one as i managed to bend a cable end when away on a trip and ended up on back of a recovery lorry.