Three other options:
1. Addi has 16" in size 13: http://www.patternworks.com/productd...LAR-NEEDLE.htm
2. Use the size 11 from knitpicks and just knit very loosely for that sections (as suzeeq said, it's probably only for the neck, so not much of the poncho).
3. Use DPNs on that section. If you do, make sure to use needles by the same company and material that you do for the rest of the poncho because changing from say, metal to bamboo, or even bamboo between different companies, can change your gauge.