Unfortunately the cables that
@momo608 recommended are for modern road or mountain bike shifters and the rounded ends will not fit the vintage Schwinn shifters without extensive filing, The Schwinn Single and Twin-Stik shifters were similar to Huret shifters and require Huret cables using much thinner "lolipop" ends. Here you will find NOS Huret cables that will fit with no modification:
http://www.ebay.com/itm/NOS-Huret-shifting-cables-63-Jubelee-Schwinn-Raleigh-/160354414190
Note that is a package of 10 cables for $12 (including shipping), so they are only $1.20 each.