I found some decent replacement laces for my chuck 70s on eBay. They're not exactly the same, but close. The new lace is on the left shoe, old lace on the right. :
Amazon.com: ZHENTOR [2 Pairs] Flat Athletic Shoe Laces - Cotton Shoelaces for Sport Canvas Shoes and 70S Replacement Sneakers (31in, Beige) : Clothing, Shoes & Jewelry
4 Ways to Lace Converse - wikiHow
Converse Classic Chucks vs. Chuck 70s: Which Pair Should You Get?