I like paracord, but braided dacron kite string is a good thing to have. My Altoids kit has paracord wrapped around the outside, as much as can fit, but it's not all THAT much. Inside I have some sewing machine bobbins filled with heavy kite line.

http://www.intothewind.com/shop/Line_and_Accessories/Bulk_Line_and_Reels_for_Traditional_Kites

Scroll down the page to find the heavier cord. You can get up to 500lb test, but you'll have to take out a loan to afford it. I'd stick to the 100lb stuff myself, unless you're planning to tow cars with it. Much cheaper. $15 for 500 feet (which is quite a bit).