For anything larger than the little 1/2A ships, you need either steel cable or single strand steel. Only the very top contest flyers mess with single strands-figure on cable.

The AMA has contest rules for line size that most people follow because they are pretty sensible rules. Here's the AMA web site page. Scroll down a little to find the table of line diameters:
http://www.modelaircraft.org/comp/05...ook/claero.pdf
Your 15 size plane can fly on .012 diameter steel cable lines, probably 52 to 58 feet long. We have a 15 size plane that my son flies and we use .015/58 foot lines because they aren't as fragile and they actually slow the plane down to a more comfortable speed.

Suppliers include Sig, Brodak, RSM Distribution, and Sullivan. Most of these are line "kits" where you get the cable and parts and need to wrap the ends. The Sullivans are ready to fly, but you have to confirm that the length on the box in what you actually get. They vary all over. Guys who make up a lot of lines buy a 1000' spool of cable. It's surprisingly cheap that way.

For length, you have to experiment a little. Base the length on how you and the plane like the way it flies. If you need some help with wrapping the ends I can find you some info. The AMA rule book under CL General has pictures of how to do it.

Kelvin