I think years ago I bought some flat speaker wire from RatShack.
Google "flat speaker wire." There are tons of hits for everything from fairly flat to superflat with an adhesive side. The prices I see are mostly reasonable depending on the length of your run (heck, even the MonsterCable flat wire for a short run doesn't break the bank), especially compared to the Nordost stuff. I don't know what "the cheapest" is as I haven't had a need for this kind of wire in a long time, nor do I know of the best source to buy it in bulk.